Many large websites use multiple programming language in their back-end depending upon the particular feature you're utilizing. The back-end of Google search is powered by Python, although they also make use of PHP, Java, and other language at various locations. (The creator of Python is employed at Google, by-the-by.)
Amazon and eBay primarily rely on CGI, which isn't a programming language, but rather an interface that allows a web application to communicate with a program on the server, such as one written in C. These (typically and predominantly) offline languages that are compiled tend to offer greatly improved processing speeds vs. interpreted languages, such as Python, PHP, or Ruby.
Microsoft-owned websites tend to rely on ASP.net, but although many would say they strictly rely on this technology, it's not true. You'll find Python code and others within Microsoft projects, as well.
Wikipedia relies predominantly on PHP, and Twitter relies mostly on Ruby on Rails, although their most intensive features are handled by Scala, due to scaling problems they've had with Ruby.
Thursday, December 10, 2009
BSD Systems
There are a number of Unix-like operating systems based on or descended from the Berkeley Software Distribution (BSD) series of Unix variants. The three most notable descendants in current use are FreeBSD, OpenBSD, and NetBSD, which are all derived from 386BSD and 4.4BSD-Lite, by various routes. Both NetBSD and FreeBSD started life in 1993, initially derived from 386BSD, but in 1994 migrating to a 4.4BSD-Lite code base. OpenBSD was forked in 1995 from NetBSD. Other notable derivatives include DragonFly BSD, which was forked from FreeBSD 4.8, and Apple Inc.'s Mac OS X, with its Darwin base including a large amount of code derived from FreeBSD.
Most of the current BSD operating systems are open source and available for download, free of charge, under the BSD License, the most notable exception being Mac OS X. They also generally use a monolithic kernel architecture, apart from Mac OS X and DragonFly BSD which feature hybrid kernels. The various open source BSD projects generally develop the kernel and userland programs and libraries together, the source code being managed using a single central source repository.
In the past, BSD was also used as a basis for several proprietary versions of UNIX, such as Sun's SunOS, Sequent's Dynix, NeXT's NeXTSTEP, DEC's Ultrix and OSF/1 AXP (now Tru64 UNIX). Of these, only the last is still currently supported in its original form. Parts of NeXT's software became the foundation for Mac OS X, among the most commercially successful BSD variants in the general market.
Most of the current BSD operating systems are open source and available for download, free of charge, under the BSD License, the most notable exception being Mac OS X. They also generally use a monolithic kernel architecture, apart from Mac OS X and DragonFly BSD which feature hybrid kernels. The various open source BSD projects generally develop the kernel and userland programs and libraries together, the source code being managed using a single central source repository.
In the past, BSD was also used as a basis for several proprietary versions of UNIX, such as Sun's SunOS, Sequent's Dynix, NeXT's NeXTSTEP, DEC's Ultrix and OSF/1 AXP (now Tru64 UNIX). Of these, only the last is still currently supported in its original form. Parts of NeXT's software became the foundation for Mac OS X, among the most commercially successful BSD variants in the general market.
Thursday, June 11, 2009
Python - File I/O - IX
Introduction
Last lesson we learnt how to load external code into our program. Without any introduction (like what I usually have), let's delve into file input and output with normal text files, and later the saving and restoring of instances of classes. (Wow, our lingo power has improved greatly!)
Opening a file
To open a text file you use, well, the open() function. Seems sensible. You pass certain parameters to open() to tell it in which way the file should be opened - 'r' for read only, 'w' for writing only (if there is an old file, it will be written over), 'a' for appending (adding things on to the end of the file) and 'r+' for both reading and writing. But less talk, lets open a file for reading (you can do this in your python idle mode). Open a normal text file. We will then print out what we read inside the file:
Code Example 1 - Opening a file
openfile = open('pathtofile', 'r')
openfile.read()
That was interesting. You'll notice a lot of '\n' symbols. These represent newlines (where you pressed enter to start a new line). The text is completely unformatted, but if you were to pass the output of openfile.read() to print (by typing print openfile.read()) it would be nicely formatted.
Seek and You Shall Find
Did you try typing in print openfile.read()? Did it fail? It likely did, and reason is because the 'cursor' has changed it's place. Cursor? What cursor? Well, a cursor that you really cannot see, but still a cursor. This invisible cursor tells the read function (and many other I/O functions) where to start from. To set where the cursor is, you use the seek() function. It is used in the form seek(offset, whence).
whence is optional, and determines where to seek from. If whence is 0, the bytes/letters are counted from the beginning. If it is 1, the bytes are counted from the current cursor position. If it is 2, then the bytes are counted from the end of the file. If nothing is put there, 0 is assumed.
offset decribes how far from whence that the cursor moves. for example:
* openfile.seek(45,0) would move the cursor to 45 bytes/letters after the beginning of the file.
* openfile.seek(10,1) would move the cursor to 10 bytes/letters after the current cursor position.
* openfile.seek(-77,2) would move the cursor to 77 bytes/letters before the end of the file (notice the - before the 77)
Try it out now. Use openfile.seek() to go to any spot in the file and then try typing print openfile.read(). It will print from the spot you seeked to. But realise that openfile.read() moves the cursor to the end of the file - you will have to seek again.
Other I/O Functions
There are many other functions that help you with dealing with files. They have many uses that empower you to do more, and make the things you can do easier. Let's have a look at tell(), readline(), readlines(), write() and close().
tell() returns where the cursor is in the file. It has no parameters, just type it in (like what the example below will show). This is infinitely useful, for knowing what you are refering to, where it is, and simple control of the cursor. To use it, type fileobjectname.tell() - where fileobjectname is the name of the file object you created when you opened the file (in openfile = open('pathtofile', 'r') the file object name is openfile).
readline() reads from where the cursor is till the end of the line. Remember that the end of the line isn't the edge of your screen - the line ends when you press enter to create a new line. This is useful for things like reading a log of events, or going through something progressively to process it. There are no parameters you have to pass to readline(), though you can optionally tell it the maximum number of bytes/letters to read by putting a number in the brackets. Use it with fileobjectname.readline().
readlines() is much like readline(), however readlines() reads all the lines from the cursor onwards, and returns a list, with each list element holding a line of code. Use it with fileobjectname.readlines(). For example, if you had the text file:
Code Example 2 - example text file
Line 1
Line 3
Line 4
Line 6
then the returned list from readlines() would be:
Table 1 - resulting list from readlines
Index Value
0 'Line 1'
1 ''
2 'Line 3'
3 'Line 4'
4 ''
5 'Line 6'
The write() function, writes to the file. How did you guess??? It writes from where the cursor is, and overwrites text in front of it - like in MS Word, where you press 'insert' and it writes over the top of old text. To utilise this most purposeful function, put a string between the brackets to write e.g. fileobjectname.write('this is a string').
close, you may figure, closes the file so that you can no longer read or write to it until you reopen in again. Simple enough. To use, you would write fileobjectname.close(). Simple!
In Python idle mode, open up a test file (or create a new one...) and play around with these functions. You can do some simple (and very inconvenient) text editing.
Mmm, Pickles
Pickles, in Python, are to be eaten. Their flavour is just to good to let programmers leave them in the fridge.
Ok, just joking there. Pickles, in Python, are objects saved to a file. An object in this case could be a variables, instance of a class, or a list, dictionary, or tuple. Other things can also be pickled, but with limits. The object can then be restored, or unpickled, later on. In other words, you are 'saving' your objects.
So how do we pickle? With the dump() function, which is inside the pickle module - so at the beginning of your program you will have to write import pickle. Simple enough? Then open an empty file, and use pickle.dump() to drop the object into that file. Let's try that:
Code Example 3 - pickletest.py
### pickletest.py
### PICKLE AN OBJECT
# import the pickle module
import pickle
# lets create something to be pickled
# How about a list?
picklelist = ['one',2,'three','four',5,'can you count?']
# now create a file
# replace filename with the file you want to create
file = open('filename', 'w')
# now let's pickle picklelist
pickle.dump(picklelist,file)
# close the file, and your pickling is complete
file.close()
The code to do this is laid out like pickle.load(object_to_pickle, file_object) where:
* object_to_pickle is the object you want to pickle (i.e. save it to file)
* file_object is the file object you want to write to (in this case, the file object is 'file')
After you close the file, open it in notepad and look at what you see. Along with some other gibblygook, you will see bits of the list we created.
Now to re-open, or unpickle, your file. to use this, we would use pickle.load():
Code Example 4 - unpickletest.py
### unpickletest.py
### unpickle file
# import the pickle module
import pickle
# now open a file for reading
# replace filename with the path to the file you created in pickletest.py
unpicklefile = open('filename', 'r')
# now load the list that we pickled into a new object
unpickledlist = pickle.load(unpicklefile)
# close the file, just for safety
unpicklefile.close()
# Try out using the list
for item in unpickledlist:
print item
Nifty, eh?
Of course, the limitation above is that we can only put in one object to a file. We could get around this by putting lots of picklable objects in a list or dictionary, and then pickling that list or dictionary. This is the quickest and easiest way, but you can do some pretty advanced stuff if you have advanced knowledge of pickle.
Which we won't cover.
Last lesson we learnt how to load external code into our program. Without any introduction (like what I usually have), let's delve into file input and output with normal text files, and later the saving and restoring of instances of classes. (Wow, our lingo power has improved greatly!)
Opening a file
To open a text file you use, well, the open() function. Seems sensible. You pass certain parameters to open() to tell it in which way the file should be opened - 'r' for read only, 'w' for writing only (if there is an old file, it will be written over), 'a' for appending (adding things on to the end of the file) and 'r+' for both reading and writing. But less talk, lets open a file for reading (you can do this in your python idle mode). Open a normal text file. We will then print out what we read inside the file:
Code Example 1 - Opening a file
openfile = open('pathtofile', 'r')
openfile.read()
That was interesting. You'll notice a lot of '\n' symbols. These represent newlines (where you pressed enter to start a new line). The text is completely unformatted, but if you were to pass the output of openfile.read() to print (by typing print openfile.read()) it would be nicely formatted.
Seek and You Shall Find
Did you try typing in print openfile.read()? Did it fail? It likely did, and reason is because the 'cursor' has changed it's place. Cursor? What cursor? Well, a cursor that you really cannot see, but still a cursor. This invisible cursor tells the read function (and many other I/O functions) where to start from. To set where the cursor is, you use the seek() function. It is used in the form seek(offset, whence).
whence is optional, and determines where to seek from. If whence is 0, the bytes/letters are counted from the beginning. If it is 1, the bytes are counted from the current cursor position. If it is 2, then the bytes are counted from the end of the file. If nothing is put there, 0 is assumed.
offset decribes how far from whence that the cursor moves. for example:
* openfile.seek(45,0) would move the cursor to 45 bytes/letters after the beginning of the file.
* openfile.seek(10,1) would move the cursor to 10 bytes/letters after the current cursor position.
* openfile.seek(-77,2) would move the cursor to 77 bytes/letters before the end of the file (notice the - before the 77)
Try it out now. Use openfile.seek() to go to any spot in the file and then try typing print openfile.read(). It will print from the spot you seeked to. But realise that openfile.read() moves the cursor to the end of the file - you will have to seek again.
Other I/O Functions
There are many other functions that help you with dealing with files. They have many uses that empower you to do more, and make the things you can do easier. Let's have a look at tell(), readline(), readlines(), write() and close().
tell() returns where the cursor is in the file. It has no parameters, just type it in (like what the example below will show). This is infinitely useful, for knowing what you are refering to, where it is, and simple control of the cursor. To use it, type fileobjectname.tell() - where fileobjectname is the name of the file object you created when you opened the file (in openfile = open('pathtofile', 'r') the file object name is openfile).
readline() reads from where the cursor is till the end of the line. Remember that the end of the line isn't the edge of your screen - the line ends when you press enter to create a new line. This is useful for things like reading a log of events, or going through something progressively to process it. There are no parameters you have to pass to readline(), though you can optionally tell it the maximum number of bytes/letters to read by putting a number in the brackets. Use it with fileobjectname.readline().
readlines() is much like readline(), however readlines() reads all the lines from the cursor onwards, and returns a list, with each list element holding a line of code. Use it with fileobjectname.readlines(). For example, if you had the text file:
Code Example 2 - example text file
Line 1
Line 3
Line 4
Line 6
then the returned list from readlines() would be:
Table 1 - resulting list from readlines
Index Value
0 'Line 1'
1 ''
2 'Line 3'
3 'Line 4'
4 ''
5 'Line 6'
The write() function, writes to the file. How did you guess??? It writes from where the cursor is, and overwrites text in front of it - like in MS Word, where you press 'insert' and it writes over the top of old text. To utilise this most purposeful function, put a string between the brackets to write e.g. fileobjectname.write('this is a string').
close, you may figure, closes the file so that you can no longer read or write to it until you reopen in again. Simple enough. To use, you would write fileobjectname.close(). Simple!
In Python idle mode, open up a test file (or create a new one...) and play around with these functions. You can do some simple (and very inconvenient) text editing.
Mmm, Pickles
Pickles, in Python, are to be eaten. Their flavour is just to good to let programmers leave them in the fridge.
Ok, just joking there. Pickles, in Python, are objects saved to a file. An object in this case could be a variables, instance of a class, or a list, dictionary, or tuple. Other things can also be pickled, but with limits. The object can then be restored, or unpickled, later on. In other words, you are 'saving' your objects.
So how do we pickle? With the dump() function, which is inside the pickle module - so at the beginning of your program you will have to write import pickle. Simple enough? Then open an empty file, and use pickle.dump() to drop the object into that file. Let's try that:
Code Example 3 - pickletest.py
### pickletest.py
### PICKLE AN OBJECT
# import the pickle module
import pickle
# lets create something to be pickled
# How about a list?
picklelist = ['one',2,'three','four',5,'can you count?']
# now create a file
# replace filename with the file you want to create
file = open('filename', 'w')
# now let's pickle picklelist
pickle.dump(picklelist,file)
# close the file, and your pickling is complete
file.close()
The code to do this is laid out like pickle.load(object_to_pickle, file_object) where:
* object_to_pickle is the object you want to pickle (i.e. save it to file)
* file_object is the file object you want to write to (in this case, the file object is 'file')
After you close the file, open it in notepad and look at what you see. Along with some other gibblygook, you will see bits of the list we created.
Now to re-open, or unpickle, your file. to use this, we would use pickle.load():
Code Example 4 - unpickletest.py
### unpickletest.py
### unpickle file
# import the pickle module
import pickle
# now open a file for reading
# replace filename with the path to the file you created in pickletest.py
unpicklefile = open('filename', 'r')
# now load the list that we pickled into a new object
unpickledlist = pickle.load(unpicklefile)
# close the file, just for safety
unpicklefile.close()
# Try out using the list
for item in unpickledlist:
print item
Nifty, eh?
Of course, the limitation above is that we can only put in one object to a file. We could get around this by putting lots of picklable objects in a list or dictionary, and then pickling that list or dictionary. This is the quickest and easiest way, but you can do some pretty advanced stuff if you have advanced knowledge of pickle.
Which we won't cover.
Python - Modules - VIII
Introduction
Last lesson we covered the killer topic of Classes. As you can remember, classes are neat combinations of variables and functions in a nice, neat package. Programming lingo calls this feature encapsulation, but reguardless of what it is called, it's a really cool feature for keeping things together so the code can be used in many instances in lots of places. Of course, you've got to ask, "how do I get my classes to many places, in many programs?". The answer is to put them into a module, to be imported into other programs.
Module? What's a Module?
A module is a python file that (generally) has only defenitions of variables, functions, and classes. For example, a module might look like this:
Code Example 1 - moduletest.py
### EXAMPLE PYTHON MODULE
# Define some variables:
numberone = 1
ageofqueen = 78
# define some functions
def printhello():
print "hello"
def timesfour(input):
print input * 4
# define a class
class Piano:
def __init__(self):
self.type = raw_input("What type of piano? ")
self.height = raw_input("What height (in feet)? ")
self.price = raw_input("How much did it cost? ")
self.age = raw_input("How old is it (in years)? ")
def printdetails(self):
print "This piano is a/an " + self.height + " foot",
print self.type, "piano, " + self.age, "years old and costing\
" + self.price + " dollars."
As you see, a module looks pretty much like your normal python program.
So what do we do with a module? We import bits of it (or all of it) into other programs.
To import all the variables, functions and classes from moduletest.py into another program you are writing, we use the import operator. For example, to import moduletest.py into your main program, you would have this:
Code Example 2 - mainprogram.py
### mainprogam.py
### IMPORTS ANOTHER MODULE
import moduletest
This assumes that the module is in the same directory as mainprogram.py, or is a default module that comes with python. You leave out the '.py' at the end of the file - it is ignored. You normally put all import statements at the beginning of the python file, but technically they can be anywhere. In order to use the items in the module in your main program, you use the following:
Code Example 3 - mainprogram.py continued
### USING AN IMPORTED MODULE
# Use the form modulename.itemname
# Examples:
print moduletest.ageofqueen
cfcpiano = moduletest.Piano()
cfcpiano.printdetails()
As you see, the modules that you import act very much like the classes we looked at last lesson - anything inside them must be preceeded with modulename. for it to work.
More module thingummyjigs (in lack of a better title)
Wish you could get rid of the modulename. part that you have to put before every item you use from a module? No? Never? Well, I'll teach it you anyway.
One way to avoid this hassle is to import only the wanted objects from the module. To do this, you use the from operator. You use it in the form of from modulename import itemname. Here is an example:
Code Example 4 - importing individual objects
### IMPORT ITEMS DIRECTLY INTO YOUR PROGRAM
# import them
from moduletest import ageofqueen
from moduletest import printhello
# now try using them
print ageofqueen
printhello()
What is the point of this? Well, maybe you could use it to make your code a little more readable. If we get into heaps of modules inside modules, it could also remove that extra layer of crypticness.
If you wanted to, you could import everything from a module is this way by using from modulename import *. Of course, this can be troublesome if there are objects in your program with the same name as some items in the module. With large modules, this can easily happen, and can cause many a headache. A better way to do this would be to import a module in the normal way (without the from operator) and then assign items to a local name:
Code Example 5 - mainprogram.py continued
### ASSIGNING ITEMS TO A LOCAL NAME
# Assigning to a local name
timesfour = moduletest.timesfour
# Using the local name
print timesfour(565)
This way, you can remove some crypticness, AND have all of the items from a certain module.
Last lesson we covered the killer topic of Classes. As you can remember, classes are neat combinations of variables and functions in a nice, neat package. Programming lingo calls this feature encapsulation, but reguardless of what it is called, it's a really cool feature for keeping things together so the code can be used in many instances in lots of places. Of course, you've got to ask, "how do I get my classes to many places, in many programs?". The answer is to put them into a module, to be imported into other programs.
Module? What's a Module?
A module is a python file that (generally) has only defenitions of variables, functions, and classes. For example, a module might look like this:
Code Example 1 - moduletest.py
### EXAMPLE PYTHON MODULE
# Define some variables:
numberone = 1
ageofqueen = 78
# define some functions
def printhello():
print "hello"
def timesfour(input):
print input * 4
# define a class
class Piano:
def __init__(self):
self.type = raw_input("What type of piano? ")
self.height = raw_input("What height (in feet)? ")
self.price = raw_input("How much did it cost? ")
self.age = raw_input("How old is it (in years)? ")
def printdetails(self):
print "This piano is a/an " + self.height + " foot",
print self.type, "piano, " + self.age, "years old and costing\
" + self.price + " dollars."
As you see, a module looks pretty much like your normal python program.
So what do we do with a module? We import bits of it (or all of it) into other programs.
To import all the variables, functions and classes from moduletest.py into another program you are writing, we use the import operator. For example, to import moduletest.py into your main program, you would have this:
Code Example 2 - mainprogram.py
### mainprogam.py
### IMPORTS ANOTHER MODULE
import moduletest
This assumes that the module is in the same directory as mainprogram.py, or is a default module that comes with python. You leave out the '.py' at the end of the file - it is ignored. You normally put all import statements at the beginning of the python file, but technically they can be anywhere. In order to use the items in the module in your main program, you use the following:
Code Example 3 - mainprogram.py continued
### USING AN IMPORTED MODULE
# Use the form modulename.itemname
# Examples:
print moduletest.ageofqueen
cfcpiano = moduletest.Piano()
cfcpiano.printdetails()
As you see, the modules that you import act very much like the classes we looked at last lesson - anything inside them must be preceeded with modulename. for it to work.
More module thingummyjigs (in lack of a better title)
Wish you could get rid of the modulename. part that you have to put before every item you use from a module? No? Never? Well, I'll teach it you anyway.
One way to avoid this hassle is to import only the wanted objects from the module. To do this, you use the from operator. You use it in the form of from modulename import itemname. Here is an example:
Code Example 4 - importing individual objects
### IMPORT ITEMS DIRECTLY INTO YOUR PROGRAM
# import them
from moduletest import ageofqueen
from moduletest import printhello
# now try using them
print ageofqueen
printhello()
What is the point of this? Well, maybe you could use it to make your code a little more readable. If we get into heaps of modules inside modules, it could also remove that extra layer of crypticness.
If you wanted to, you could import everything from a module is this way by using from modulename import *. Of course, this can be troublesome if there are objects in your program with the same name as some items in the module. With large modules, this can easily happen, and can cause many a headache. A better way to do this would be to import a module in the normal way (without the from operator) and then assign items to a local name:
Code Example 5 - mainprogram.py continued
### ASSIGNING ITEMS TO A LOCAL NAME
# Assigning to a local name
timesfour = moduletest.timesfour
# Using the local name
print timesfour(565)
This way, you can remove some crypticness, AND have all of the items from a certain module.
Wednesday, June 10, 2009
Python - Classes - VII
Introduction
One thing that you will get to know about programming, is that programmers like to be lazy. If something has been done before, why should you do it again?
That is what functions cover in python. You've already had your code do something special. Now you want to do it again. You put that special code into a function, and re-use it for all it is worth. You can refer to a function anywhere in your code, and the computer will always know what you are talking about. Handy, eh?
Of course, functions have their limitations. Functions don't store any information like variables do - every time a function is run, it starts afresh. However, certain functions and variables are related to each other very closely, and need to interact with each other a lot. For example, imagine you have a golf club. It has information about it (i.e. variables) like the length of the shaft, the material of the grip, and the material of the head. It also has functions associated with it, like the function of swinging your golf club, or the function of breaking it in pure frustration. For those functions, you need to know the variables of the shaft length, head material, etc.
That can easily be worked around with normal functions. Parameters affect the effect of a function. But what if a function needs to affect variables? What happens if each time you use your golf club, the shaft gets weaker, the grip on the handle wears away a little, you get that little more frustrated, and a new scratch is formed on the head of the club? A function cannot do that. A function only makes one output, not four or five, or five hundred. What is needed is a way to group functions and variables that are closely related into one place so that they can interact with each other.
Chances are that you also have more than one golf club. Without classes, you need to write a whole heap of code for each different golf club. This is a pain, seeing that all clubs share common features, it is just that some have changed properties - like what the shaft is made of, and it's weight. The ideal situation would be to have a design of your basic golf club. Each time you create a new club, simply specify its attributes - the length of its shaft, its weight, etc.
Or what if you want a golf club, which has added extra features? Maybe you decide to attach a clock to your golf club (why, I don't know - it was your idea). Does this mean that we have to create this golf club from scratch? We would have to write code first for our basic golf club, plus all of that again, and the code for the clock, for our new design. Wouldn't it be better if we were to just take our existing golf club, and then tack the code for the clock to it?
These problems that a thing called object-oriented-programming solves. It puts functions and variables together in a way that they can see each other and work together, be replicated, and altered as needed, and not when unneeded. And we use a thing called a 'class' to do this.
Creating a Class
What is a class? Think of a class as a blueprint. It isn't something in itself, it simply describes how to make something. You can create lots of objects from that blueprint - known technically as an instance.
So how do you make these so-called 'classes'? very easily, with the class operator:
Code Example 1 - defining a class
# Defining a class
class class_name:
[statement 1]
[statement 2]
[statement 3]
[etc]
Makes little sense? Thats ok, here is an example, that creates the definition of a Shape:
Code Example 2 - Example of a Class
#An example of a class
class Shape:
def __init__(self,x,y):
self.x = x
self.y = y
description = "This shape has not been described yet"
author = "Nobody has claimed to make this shape yet"
def area(self):
return self.x * self.y
def perimeter(self):
return 2 * self.x + 2 * self.y
def describe(self,text):
self.description = text
def authorName(self,text):
self.author = text
def scaleSize(self,scale):
self.x = self.x * scale
self.y = self.y * scale
What you have created is a description of a shape (That is, the variables) and what operations you can do with the shape (That is, the fuctions). This is very important - you have not made an actual shape, simply the description of what a shape is. The shape has a width (x), a height (y), and an area and perimeter (area(self) and perimeter(self)). No code is run when you define a class - you are simply making functions and variables.
The function called __init__ is run when we create an instance of Shape - that is, when we create an actual shape, as opposed to the 'blueprint' we have here, __init__ is run. You will understand how this works later.
self is how we refer to things in the class from within itself. self is the first parameter in any function defined inside a class. Any function or variable created on the first level of indentation (that is, lines of code that start one TAB to the right of where we put class Shape is automatically put into self. To access these functions and variables elsewhere inside the class, their name must be preceeded with self and a full-stop (e.g. self.variable_name).
Using a class
Its all well and good that we can make a class, but how do we use one? Here is an example, of what we call creating an instance of a class. Assume that the code example 2 has already been run:
Code Example 3 - Creating a class
rectangle = Shape(100,45)
What has been done? It takes a little explaining...
The __init__ function really comes into play at this time. We create an instance of a class by first giving its name (in this case, Shape) and then, in brackets, the values to pass to the __init__ function. The init function runs (using the parameters you gave it in brackets) and then spits out an instance of that class, which in this case is assigned to the name rectangle.
Think of our class instance, rectangle, as a self-contained collection of variables and functions. In the same way that we used self to access functions and variables of the class instance from within itself, we use the name that we assigned to it now (rectangle) to access functions and variables of the class instance from outside of itself. Following on from the code we ran above, we would do this:
Code Example 4 - accessing attributes from outside an instance
#finding the area of your rectangle:
print rectangle.area()
#finding the perimeter of your rectangle:
print rectangle.perimeter()
#describing the rectangle
rectangle.describe("A wide rectangle, more than twice\
as wide as it is tall")
#making the rectangle 50% smaller
rectangle.scaleSize(0.5)
#re-printing the new area of the rectangle
print rectangle.area()
As you see, where self would be used from within the class instance, its assigned name is used when outside the class. We do this to view and change the variables inside the class, and to access the functions that are there.
We aren't limited to a single instance of a class - we could have as many instances as we like. I could do this:
Code Example 5 - More than one instance
longrectangle = Shape(120,10)
fatrectangle = Shape(130,120)
and both longrectangle and fatrectangle have their own functions and variables contained inside them - they are totally independent of each other. There is no limit to the number of instances I could create.
Lingo
Object-oriented-programming has a set of lingo that is associated with it. Its about time that we have this all cleared up:
* When we first describe a class, we are defining it (like with functions)
* The ability to group similar functions and variables together is called encapsulation
* The word 'class' can be used when describing the code where the class is defined (like how a function is defined), and it can also refer to an instance of that class - this can get confusing, so make sure you know in which form we are talking about classes
* A variable inside a class is known as an Attribute
* A function inside a class is known as a method
* A class is in the same category of things as variables, lists, dictionaries, etc. That is, they are objects
* A class is known as a 'data structure' - it holds data, and the methods to process that data.
Inheritance
Lets have a look back at the introduction. We know how classes group together variables and functions, known as attributes and methods, so that both the data and the code to process it is in the same spot. We can create any number of instances of that class, so that we don't have to write new code for every new object we create. But what about adding extra features to our golf club design? This is where inheritance comes into play.
Python makes inheritance really easily. We define a new class, based on another, 'parent' class. Our new class brings everything over from the parent, and we can also add other things to it. If any new attributes or methods have the same name as an attribute or method in our parent class, it is used instead of the parent one. Remember the Shape class?
Code Example 6 - the Shape class
class Shape:
def __init__(self,x,y):
self.x = x
self.y = y
description = "This shape has not been described yet"
author = "Nobody has claimed to make this shape yet"
def area(self):
return self.x * self.y
def perimeter(self):
return 2 * self.x + 2 * self.y
def describe(self,text):
self.description = text
def authorName(self,text):
self.author = text
def scaleSize(self,scale):
self.x = self.x * scale
self.y = self.y * scale
If we wanted to define a new class, lets say a square, based on our previous Shape class, we would do this:
Code Example 7 - Using inheritance
class Square(Shape):
def __init__(self,x):
self.x = x
self.y = x
It is just like normally defining a class, but this time we put in brackets after the name, the parent class that we inherited from. As you see, we described a square really quickly because of this. That's because we inherited everything from the shape class, and changed only what needed to be changed. In this case we redefined the __init__ function of Shape so that the X and Y values are the same.
Let's take from what we have learnt, and create another new class, this time inherited from Square. It will be two squares, one immediately left of the other:
Code Example 8 - DoubleSquare class
# The shape looks like this:
# _________
#| | |
#| | |
#|____|____|
class DoubleSquare(Square):
def __init__(self,y):
self.x = 2 * y
self.y = y
def perimeter(self):
return 2 * self.x + 3 * self.y
This time, we also had to redefine the perimeter function, as there is a line going down the middle of the shape. Try creating an instance of this class. As a helpful hint, the idle command line starts where your code ends - so typing a line of code is like adding that line to the end of the program you have written.
Pointers and Dictionaries of Classes
Thinking back, when you say that one variable equals another, e.g. variable2 = variable1, the variable on the left-hand side of the equal-sign takes on the value of the variable on the right. With class instances, this happens a little differently - the name on the left becomes the class instance on the right. So in instance2 = instance1, instance2 is 'pointing' to instance1 - there are two names given to the one class instance, and you can access the class instance via either name.
In other languages, you do things like this using pointers, however in python this all happens behind the scenes.
The final thing that we will cover is dictionaries of classes. Keeping in mind what we have just learnt about pointers, we can assign an instance of a class to an entry in a list or dictionary. This allows for virtually any amount of class instances to exist when our program is run. Lets have a look at the example below, and see how it describes what I am talking about:
Code Example 9 - Dictionaries of Classes
# Again, assume the definitions on Shape,
# Square and DoubleSquare have been run.
# First, create a dictionary:
dictionary = {}
# Then, create some instances of classes in the dictionary:
dictionary["DoubleSquare 1"] = DoubleSquare(5)
dictionary["long rectangle"] = Shape(600,45)
#You can now use them like a normal class:
print dictionary["long rectangle"].area()
dictionary["DoubleSquare 1"].authorName("The Gingerbread Man")
print dictionary["DoubleSquare 1"].author
As you see, we simply replaced our boring old name on the left-hand side with an exciting, new, dynamic, dictionary entry. Pretty cool, eh?
One thing that you will get to know about programming, is that programmers like to be lazy. If something has been done before, why should you do it again?
That is what functions cover in python. You've already had your code do something special. Now you want to do it again. You put that special code into a function, and re-use it for all it is worth. You can refer to a function anywhere in your code, and the computer will always know what you are talking about. Handy, eh?
Of course, functions have their limitations. Functions don't store any information like variables do - every time a function is run, it starts afresh. However, certain functions and variables are related to each other very closely, and need to interact with each other a lot. For example, imagine you have a golf club. It has information about it (i.e. variables) like the length of the shaft, the material of the grip, and the material of the head. It also has functions associated with it, like the function of swinging your golf club, or the function of breaking it in pure frustration. For those functions, you need to know the variables of the shaft length, head material, etc.
That can easily be worked around with normal functions. Parameters affect the effect of a function. But what if a function needs to affect variables? What happens if each time you use your golf club, the shaft gets weaker, the grip on the handle wears away a little, you get that little more frustrated, and a new scratch is formed on the head of the club? A function cannot do that. A function only makes one output, not four or five, or five hundred. What is needed is a way to group functions and variables that are closely related into one place so that they can interact with each other.
Chances are that you also have more than one golf club. Without classes, you need to write a whole heap of code for each different golf club. This is a pain, seeing that all clubs share common features, it is just that some have changed properties - like what the shaft is made of, and it's weight. The ideal situation would be to have a design of your basic golf club. Each time you create a new club, simply specify its attributes - the length of its shaft, its weight, etc.
Or what if you want a golf club, which has added extra features? Maybe you decide to attach a clock to your golf club (why, I don't know - it was your idea). Does this mean that we have to create this golf club from scratch? We would have to write code first for our basic golf club, plus all of that again, and the code for the clock, for our new design. Wouldn't it be better if we were to just take our existing golf club, and then tack the code for the clock to it?
These problems that a thing called object-oriented-programming solves. It puts functions and variables together in a way that they can see each other and work together, be replicated, and altered as needed, and not when unneeded. And we use a thing called a 'class' to do this.
Creating a Class
What is a class? Think of a class as a blueprint. It isn't something in itself, it simply describes how to make something. You can create lots of objects from that blueprint - known technically as an instance.
So how do you make these so-called 'classes'? very easily, with the class operator:
Code Example 1 - defining a class
# Defining a class
class class_name:
[statement 1]
[statement 2]
[statement 3]
[etc]
Makes little sense? Thats ok, here is an example, that creates the definition of a Shape:
Code Example 2 - Example of a Class
#An example of a class
class Shape:
def __init__(self,x,y):
self.x = x
self.y = y
description = "This shape has not been described yet"
author = "Nobody has claimed to make this shape yet"
def area(self):
return self.x * self.y
def perimeter(self):
return 2 * self.x + 2 * self.y
def describe(self,text):
self.description = text
def authorName(self,text):
self.author = text
def scaleSize(self,scale):
self.x = self.x * scale
self.y = self.y * scale
What you have created is a description of a shape (That is, the variables) and what operations you can do with the shape (That is, the fuctions). This is very important - you have not made an actual shape, simply the description of what a shape is. The shape has a width (x), a height (y), and an area and perimeter (area(self) and perimeter(self)). No code is run when you define a class - you are simply making functions and variables.
The function called __init__ is run when we create an instance of Shape - that is, when we create an actual shape, as opposed to the 'blueprint' we have here, __init__ is run. You will understand how this works later.
self is how we refer to things in the class from within itself. self is the first parameter in any function defined inside a class. Any function or variable created on the first level of indentation (that is, lines of code that start one TAB to the right of where we put class Shape is automatically put into self. To access these functions and variables elsewhere inside the class, their name must be preceeded with self and a full-stop (e.g. self.variable_name).
Using a class
Its all well and good that we can make a class, but how do we use one? Here is an example, of what we call creating an instance of a class. Assume that the code example 2 has already been run:
Code Example 3 - Creating a class
rectangle = Shape(100,45)
What has been done? It takes a little explaining...
The __init__ function really comes into play at this time. We create an instance of a class by first giving its name (in this case, Shape) and then, in brackets, the values to pass to the __init__ function. The init function runs (using the parameters you gave it in brackets) and then spits out an instance of that class, which in this case is assigned to the name rectangle.
Think of our class instance, rectangle, as a self-contained collection of variables and functions. In the same way that we used self to access functions and variables of the class instance from within itself, we use the name that we assigned to it now (rectangle) to access functions and variables of the class instance from outside of itself. Following on from the code we ran above, we would do this:
Code Example 4 - accessing attributes from outside an instance
#finding the area of your rectangle:
print rectangle.area()
#finding the perimeter of your rectangle:
print rectangle.perimeter()
#describing the rectangle
rectangle.describe("A wide rectangle, more than twice\
as wide as it is tall")
#making the rectangle 50% smaller
rectangle.scaleSize(0.5)
#re-printing the new area of the rectangle
print rectangle.area()
As you see, where self would be used from within the class instance, its assigned name is used when outside the class. We do this to view and change the variables inside the class, and to access the functions that are there.
We aren't limited to a single instance of a class - we could have as many instances as we like. I could do this:
Code Example 5 - More than one instance
longrectangle = Shape(120,10)
fatrectangle = Shape(130,120)
and both longrectangle and fatrectangle have their own functions and variables contained inside them - they are totally independent of each other. There is no limit to the number of instances I could create.
Lingo
Object-oriented-programming has a set of lingo that is associated with it. Its about time that we have this all cleared up:
* When we first describe a class, we are defining it (like with functions)
* The ability to group similar functions and variables together is called encapsulation
* The word 'class' can be used when describing the code where the class is defined (like how a function is defined), and it can also refer to an instance of that class - this can get confusing, so make sure you know in which form we are talking about classes
* A variable inside a class is known as an Attribute
* A function inside a class is known as a method
* A class is in the same category of things as variables, lists, dictionaries, etc. That is, they are objects
* A class is known as a 'data structure' - it holds data, and the methods to process that data.
Inheritance
Lets have a look back at the introduction. We know how classes group together variables and functions, known as attributes and methods, so that both the data and the code to process it is in the same spot. We can create any number of instances of that class, so that we don't have to write new code for every new object we create. But what about adding extra features to our golf club design? This is where inheritance comes into play.
Python makes inheritance really easily. We define a new class, based on another, 'parent' class. Our new class brings everything over from the parent, and we can also add other things to it. If any new attributes or methods have the same name as an attribute or method in our parent class, it is used instead of the parent one. Remember the Shape class?
Code Example 6 - the Shape class
class Shape:
def __init__(self,x,y):
self.x = x
self.y = y
description = "This shape has not been described yet"
author = "Nobody has claimed to make this shape yet"
def area(self):
return self.x * self.y
def perimeter(self):
return 2 * self.x + 2 * self.y
def describe(self,text):
self.description = text
def authorName(self,text):
self.author = text
def scaleSize(self,scale):
self.x = self.x * scale
self.y = self.y * scale
If we wanted to define a new class, lets say a square, based on our previous Shape class, we would do this:
Code Example 7 - Using inheritance
class Square(Shape):
def __init__(self,x):
self.x = x
self.y = x
It is just like normally defining a class, but this time we put in brackets after the name, the parent class that we inherited from. As you see, we described a square really quickly because of this. That's because we inherited everything from the shape class, and changed only what needed to be changed. In this case we redefined the __init__ function of Shape so that the X and Y values are the same.
Let's take from what we have learnt, and create another new class, this time inherited from Square. It will be two squares, one immediately left of the other:
Code Example 8 - DoubleSquare class
# The shape looks like this:
# _________
#| | |
#| | |
#|____|____|
class DoubleSquare(Square):
def __init__(self,y):
self.x = 2 * y
self.y = y
def perimeter(self):
return 2 * self.x + 3 * self.y
This time, we also had to redefine the perimeter function, as there is a line going down the middle of the shape. Try creating an instance of this class. As a helpful hint, the idle command line starts where your code ends - so typing a line of code is like adding that line to the end of the program you have written.
Pointers and Dictionaries of Classes
Thinking back, when you say that one variable equals another, e.g. variable2 = variable1, the variable on the left-hand side of the equal-sign takes on the value of the variable on the right. With class instances, this happens a little differently - the name on the left becomes the class instance on the right. So in instance2 = instance1, instance2 is 'pointing' to instance1 - there are two names given to the one class instance, and you can access the class instance via either name.
In other languages, you do things like this using pointers, however in python this all happens behind the scenes.
The final thing that we will cover is dictionaries of classes. Keeping in mind what we have just learnt about pointers, we can assign an instance of a class to an entry in a list or dictionary. This allows for virtually any amount of class instances to exist when our program is run. Lets have a look at the example below, and see how it describes what I am talking about:
Code Example 9 - Dictionaries of Classes
# Again, assume the definitions on Shape,
# Square and DoubleSquare have been run.
# First, create a dictionary:
dictionary = {}
# Then, create some instances of classes in the dictionary:
dictionary["DoubleSquare 1"] = DoubleSquare(5)
dictionary["long rectangle"] = Shape(600,45)
#You can now use them like a normal class:
print dictionary["long rectangle"].area()
dictionary["DoubleSquare 1"].authorName("The Gingerbread Man")
print dictionary["DoubleSquare 1"].author
As you see, we simply replaced our boring old name on the left-hand side with an exciting, new, dynamic, dictionary entry. Pretty cool, eh?
Tuesday, June 9, 2009
Python - Tuples, Lists, and Dictionaries - VI
Introduction
Your brain still hurting from the last lesson? Never worry, this one will require a little less thought. We're going back to something simple - variables - but a little more in depth.
Think about it - variables store one bit of information. They may regurgitate (just not on the carpet...) that information at any point, and their bit of information can be changed at any time. Variables are great at what they do - storing a piece of information that may change over time.
But what if you need to store a long list of information, which doesn't change over time? Say, for example, the names of the months of the year. Or maybe a long list of information, that does change over time? Say, for example, the names of all your cats. You might get new cats, some may die, some may become your dinner (we should trade recipies!). What about a phone book? For that you need to do a bit of referencing - you would have a list of names, and attached to each of those names, a phone number. How would you do that?
The Solution - Lists, Tuples, and Dictionaries
For these three problems, Python uses three different solutions - Tuples, lists, and dictionaries:
* Lists are what they seem - a list of values. Each one of them is numbered, starting from zero - the first one is numbered zero, the second 1, the third 2, etc. You can remove values from the list, and add new values to the end. Example: Your many cats' names.
* Tuples are just like lists, but you can't change their values. The values that you give it first up, are the values that you are stuck with for the rest of the program. Again, each value is numbered starting from zero, for easy reference. Example: the names of the months of the year.
* Dictionaries are similar to what their name suggests - a dictionary. In a dictionary, you have an 'index' of words, and for each of them a definition. In python, the word is called a 'key', and the definition a 'value'. The values in a dictionary aren't numbered - tare similar to what their name suggests - a dictionary. In a dictionary, you have an 'index' of words, and for each of them a definition. In python, the word is called a 'key', and the definition a 'value'. The values in a dictionary aren't numbered - they aren't in any specific order, either - the key does the same thing. You can add, remove, and modify the values in dictionaries. Example: telephone book.
Tuples
Tuples are pretty easy to make. You give your tuple a name, then after that the list of values it will carry. For example, the months of the year:
Code Example 1 - creating a tuple
months = ('January','February','March','April','May','June',\
'July','August','September','October','November',' December')
* Note that the '\' thingy at the end of sthurlow.comthe first line carries over that line of code to the next line. It is usefull way of making big lines more readable.
* Technically you don't have to put those parentheses there (the '(' and ')' thingies) but it stops python from getting things confused.
* You may have spaces after the commas if you feel it necessary - it doesn't really matter.
Python then organises those values in a handy, numbered index - starting from zero, in the order that you entered them in. It would be organised like this:
Table 1 - tuple indicies
Index Value
0 January
1 Feb
2 Mar
3 Apr
4 May
5 Jun
6 Jul
7 Aug
8 Sep
9 Oct
10 Nov
11 Dec
And that is tuples! Really easy...
Lists
Lists are extremely similar to tuples. Lists are modifiable (or 'mutable', as a programmer may say), so their values can be changed. Most of the time we use lists, not tuples, because we want to easily change the values of things if we need to.
Lists are defined very similarly to tuples. Say you have FIVE cats, called Tom, Snappy, Kitty, Jessie and Chester. To put them in a list, you would do this:
Code Example 2 - Creating a List
cats = ['Tom', 'Snappy', 'Kitty', 'Jessie', 'Chester']
As you see, the code is exactly the same as a tuple, EXCEPT that all the values are put between square brackets, not parentheses. Again, you don't have to have spaces after the comma.
You recall values from lists exactly the same as you do with tuples. For example, to print the name of your 3rd cat you would do this:
Code Example 3 - Recalling items from a list
print cats[2]
You can also recall a range of examples, like above, for example - cats[0:2] would recall your 1st and 2nd cats.
Where lists come into their own is how they can be modified. To add a value to a list, you use the 'append()' function. Let's say you got a new cat called Catherine. To add her to the list you'd do this:
Code Example 4 - Adding items to a list
cats.append('Catherine')
That's a little weird, isn't it? I'll explain. That function is in a funny spot - after a period (the '.' kind of period, not otherwise), after the list name. You'll get to see those things more in a later lesson. For the meanwhile, this is the form of the function that adds a new value to a list:
Code Example 5 - Using the append function
#add a new value to the end of a list:
list_name.append(value-to-add)
#e.g. to add the number 5038 to the list 'numbers':
numbers.append(5038)
Clears things up? Good!
Now to a sad situation - Snappy was shot by a neighbour, and eaten for their dinner (good on 'em!). You need to remove him (or her) from the list. Removing that sorry cat is an easy task, thankfully, so you have to wallow in sadness for as short a time as possible:
Code Example 6 - Deleting an item
#Remove your 2nd cat, Snappy. Woe is you.
del cats[1]
You've just removed the 2nd cat in your list - poor old Snappy.
And with that morbid message, lets move on to...
Dictionaries
Ok, so there is more to life than the names of your cats. You need to call your sister, mother, son, the fruit man, and anyone else who needs to know that their favourite cat is dead. For that you need a telephone book.
Now, the lists we've used above aren't really suitable for a telephone book. You need to know a number based on someone's name - not the other way around, like what we did with the cats. In the examples of months and cats, we gave the computer a number, and it gave us a name. This time we want to give the computer a name, and it give us a number. For this we need Dictionaries.
So how do we make a dictionary? Put away your binding equipment, it isn't that advanced.
Remember, dictionaries have keys, and values. In a phone book, you have people's names, then their numbers. See a similarity?
When you initially create a dictionary, it is very much like making a tuple or list. Tuples have ( and ) things, lists have [ and ] things. Guess what! dictionaries have { and } things - curly braces. Here is an example below, showing a dictionary with four phone numbers in it:
Code Example 7 - Creating a dictionary
#Make the phone book:
phonebook = {'Andrew Parson':8806336, \
'Emily Everett':6784346, 'Peter Power':7658344, \
'Lewis Lame':1122345}
the program would then print Lewis Lame's number onscreen. Notice how instead of identifying the value by a number, like in the cats and months examples, we identify the value, using another value - in this case the person's name.
Ok, you've created a new phone book. Now you want to add new numbers to the book. What do you do? A very simple line of code:
Code Example 8 - Adding entries to a dictionary
#Add the person 'Gingerbread Man' to the phonebook:
phonebook['Gingerbread Man'] = 1234567
# Didn't think I would give you
# my real number now, would I?
All that line is saying is that there is a person called Gingerbread Man in the phone book, and his number is 1234567. In other words - the key is 'Gingerbread Man', and the value is 1234567.
You delete entries in a dictionary just like in a list. Let's say Andrew Parson is your neighbour, and shot your cat. You never want to talk to him again, and therefore don't need his number. Just like in a list, you'd do this:
Code Example 9 - Removing entries from a dictionary
del phonebook['Andrew Parson']
Again, very easy. the 'del' operator deletes any function, variable, or entry in a list or dictionary (An entry in a dictionary is just a variable with a number or text string as a name. This comes in handy later on.)
remember that append function that we used with the list? Well, there are quite a few of those that can be used with dictionaries. Below, I will write you a program, and it will incorporate some of those functions in. It will have comments along the way explaining what it does.
Type this program into Python IDLE (you can skip the comments). Experiment as much as you like with it. Type it where you see the lines beginning with >>>
Code Example 10 - Functions of dictionaries
#A few examples of a dictionary
#First we define the dictionary
#it will have nothing in it this time
ages = {}
#Add a couple of names to the dictionary
ages['Sue'] = 23
ages['Peter'] = 19
ages['Andrew'] = 78
ages['Karren'] = 45
#Use the function has_key() -
#This function takes this form:
#function_name.has_key(key-name)
#It returns TRUE
#if the dictionary has key-name in it
#but returns FALSE if it doesn't.
#Remember - this is how 'if' statements work -
#they run if something is true
#and they don't when something is false.
if ages.has_key('Sue'):
print "Sue is in the dictionary. She is", \
ages['Sue'], "years old"
else:
print "Sue is not in the dictionary"
#Use the function keys() -
#This function returns a list
#of all the names of the keys.
#E.g.
print "The following people are in the dictionary:"
print ages.keys()
#You could use this function to
#put all the key names in a list:
keys = ages.keys()
#You can also get a list
#of all the values in a dictionary.
#You use the values() function:
print "People are aged the following:", \
ages.values()
#Put it in a list:
values = ages.values()
#You can sort lists, with the sort() function
#It will sort all values in a list
#alphabetically, numerically, etc...
#You can't sort dictionaries -
#they are in no particular order
print keys
keys.sort()
print keys
print values
values.sort()
print values
#You can find the number of entries
#with the len() function:
print "The dictionary has", \
len(ages), "entries in it"
Your brain still hurting from the last lesson? Never worry, this one will require a little less thought. We're going back to something simple - variables - but a little more in depth.
Think about it - variables store one bit of information. They may regurgitate (just not on the carpet...) that information at any point, and their bit of information can be changed at any time. Variables are great at what they do - storing a piece of information that may change over time.
But what if you need to store a long list of information, which doesn't change over time? Say, for example, the names of the months of the year. Or maybe a long list of information, that does change over time? Say, for example, the names of all your cats. You might get new cats, some may die, some may become your dinner (we should trade recipies!). What about a phone book? For that you need to do a bit of referencing - you would have a list of names, and attached to each of those names, a phone number. How would you do that?
The Solution - Lists, Tuples, and Dictionaries
For these three problems, Python uses three different solutions - Tuples, lists, and dictionaries:
* Lists are what they seem - a list of values. Each one of them is numbered, starting from zero - the first one is numbered zero, the second 1, the third 2, etc. You can remove values from the list, and add new values to the end. Example: Your many cats' names.
* Tuples are just like lists, but you can't change their values. The values that you give it first up, are the values that you are stuck with for the rest of the program. Again, each value is numbered starting from zero, for easy reference. Example: the names of the months of the year.
* Dictionaries are similar to what their name suggests - a dictionary. In a dictionary, you have an 'index' of words, and for each of them a definition. In python, the word is called a 'key', and the definition a 'value'. The values in a dictionary aren't numbered - tare similar to what their name suggests - a dictionary. In a dictionary, you have an 'index' of words, and for each of them a definition. In python, the word is called a 'key', and the definition a 'value'. The values in a dictionary aren't numbered - they aren't in any specific order, either - the key does the same thing. You can add, remove, and modify the values in dictionaries. Example: telephone book.
Tuples
Tuples are pretty easy to make. You give your tuple a name, then after that the list of values it will carry. For example, the months of the year:
Code Example 1 - creating a tuple
months = ('January','February','March','April','May','June',\
'July','August','September','October','November',' December')
* Note that the '\' thingy at the end of sthurlow.comthe first line carries over that line of code to the next line. It is usefull way of making big lines more readable.
* Technically you don't have to put those parentheses there (the '(' and ')' thingies) but it stops python from getting things confused.
* You may have spaces after the commas if you feel it necessary - it doesn't really matter.
Python then organises those values in a handy, numbered index - starting from zero, in the order that you entered them in. It would be organised like this:
Table 1 - tuple indicies
Index Value
0 January
1 Feb
2 Mar
3 Apr
4 May
5 Jun
6 Jul
7 Aug
8 Sep
9 Oct
10 Nov
11 Dec
And that is tuples! Really easy...
Lists
Lists are extremely similar to tuples. Lists are modifiable (or 'mutable', as a programmer may say), so their values can be changed. Most of the time we use lists, not tuples, because we want to easily change the values of things if we need to.
Lists are defined very similarly to tuples. Say you have FIVE cats, called Tom, Snappy, Kitty, Jessie and Chester. To put them in a list, you would do this:
Code Example 2 - Creating a List
cats = ['Tom', 'Snappy', 'Kitty', 'Jessie', 'Chester']
As you see, the code is exactly the same as a tuple, EXCEPT that all the values are put between square brackets, not parentheses. Again, you don't have to have spaces after the comma.
You recall values from lists exactly the same as you do with tuples. For example, to print the name of your 3rd cat you would do this:
Code Example 3 - Recalling items from a list
print cats[2]
You can also recall a range of examples, like above, for example - cats[0:2] would recall your 1st and 2nd cats.
Where lists come into their own is how they can be modified. To add a value to a list, you use the 'append()' function. Let's say you got a new cat called Catherine. To add her to the list you'd do this:
Code Example 4 - Adding items to a list
cats.append('Catherine')
That's a little weird, isn't it? I'll explain. That function is in a funny spot - after a period (the '.' kind of period, not otherwise), after the list name. You'll get to see those things more in a later lesson. For the meanwhile, this is the form of the function that adds a new value to a list:
Code Example 5 - Using the append function
#add a new value to the end of a list:
list_name.append(value-to-add)
#e.g. to add the number 5038 to the list 'numbers':
numbers.append(5038)
Clears things up? Good!
Now to a sad situation - Snappy was shot by a neighbour, and eaten for their dinner (good on 'em!). You need to remove him (or her) from the list. Removing that sorry cat is an easy task, thankfully, so you have to wallow in sadness for as short a time as possible:
Code Example 6 - Deleting an item
#Remove your 2nd cat, Snappy. Woe is you.
del cats[1]
You've just removed the 2nd cat in your list - poor old Snappy.
And with that morbid message, lets move on to...
Dictionaries
Ok, so there is more to life than the names of your cats. You need to call your sister, mother, son, the fruit man, and anyone else who needs to know that their favourite cat is dead. For that you need a telephone book.
Now, the lists we've used above aren't really suitable for a telephone book. You need to know a number based on someone's name - not the other way around, like what we did with the cats. In the examples of months and cats, we gave the computer a number, and it gave us a name. This time we want to give the computer a name, and it give us a number. For this we need Dictionaries.
So how do we make a dictionary? Put away your binding equipment, it isn't that advanced.
Remember, dictionaries have keys, and values. In a phone book, you have people's names, then their numbers. See a similarity?
When you initially create a dictionary, it is very much like making a tuple or list. Tuples have ( and ) things, lists have [ and ] things. Guess what! dictionaries have { and } things - curly braces. Here is an example below, showing a dictionary with four phone numbers in it:
Code Example 7 - Creating a dictionary
#Make the phone book:
phonebook = {'Andrew Parson':8806336, \
'Emily Everett':6784346, 'Peter Power':7658344, \
'Lewis Lame':1122345}
the program would then print Lewis Lame's number onscreen. Notice how instead of identifying the value by a number, like in the cats and months examples, we identify the value, using another value - in this case the person's name.
Ok, you've created a new phone book. Now you want to add new numbers to the book. What do you do? A very simple line of code:
Code Example 8 - Adding entries to a dictionary
#Add the person 'Gingerbread Man' to the phonebook:
phonebook['Gingerbread Man'] = 1234567
# Didn't think I would give you
# my real number now, would I?
All that line is saying is that there is a person called Gingerbread Man in the phone book, and his number is 1234567. In other words - the key is 'Gingerbread Man', and the value is 1234567.
You delete entries in a dictionary just like in a list. Let's say Andrew Parson is your neighbour, and shot your cat. You never want to talk to him again, and therefore don't need his number. Just like in a list, you'd do this:
Code Example 9 - Removing entries from a dictionary
del phonebook['Andrew Parson']
Again, very easy. the 'del' operator deletes any function, variable, or entry in a list or dictionary (An entry in a dictionary is just a variable with a number or text string as a name. This comes in handy later on.)
remember that append function that we used with the list? Well, there are quite a few of those that can be used with dictionaries. Below, I will write you a program, and it will incorporate some of those functions in. It will have comments along the way explaining what it does.
Type this program into Python IDLE (you can skip the comments). Experiment as much as you like with it. Type it where you see the lines beginning with >>>
Code Example 10 - Functions of dictionaries
#A few examples of a dictionary
#First we define the dictionary
#it will have nothing in it this time
ages = {}
#Add a couple of names to the dictionary
ages['Sue'] = 23
ages['Peter'] = 19
ages['Andrew'] = 78
ages['Karren'] = 45
#Use the function has_key() -
#This function takes this form:
#function_name.has_key(key-name)
#It returns TRUE
#if the dictionary has key-name in it
#but returns FALSE if it doesn't.
#Remember - this is how 'if' statements work -
#they run if something is true
#and they don't when something is false.
if ages.has_key('Sue'):
print "Sue is in the dictionary. She is", \
ages['Sue'], "years old"
else:
print "Sue is not in the dictionary"
#Use the function keys() -
#This function returns a list
#of all the names of the keys.
#E.g.
print "The following people are in the dictionary:"
print ages.keys()
#You could use this function to
#put all the key names in a list:
keys = ages.keys()
#You can also get a list
#of all the values in a dictionary.
#You use the values() function:
print "People are aged the following:", \
ages.values()
#Put it in a list:
values = ages.values()
#You can sort lists, with the sort() function
#It will sort all values in a list
#alphabetically, numerically, etc...
#You can't sort dictionaries -
#they are in no particular order
print keys
keys.sort()
print keys
print values
values.sort()
print values
#You can find the number of entries
#with the len() function:
print "The dictionary has", \
len(ages), "entries in it"
Python - Functions - V
Introduction
Last lesson I said that we would delve into purposefull programming. That involves user input, and user input requires a thing called functions.
What are functions? Well, in effect, functions are little self-contained programs that perform a specific task, which you can incorporate into your own, larger programs. After you have created a function, you can use it at any time, in any place. This saves you the time and effort of having to retell the computer what to do every time it does a common task, for example getting the user to type something in.
Using a function
Python has lots of pre-made functions, that you can use right now, simply by 'calling' them. 'Calling' a function involves you giving a function input, and it will return a value (like a variable would) as output. Don't understand? Here is the general form that calling a function takes:
Code Example 1 - How to call a function
function_name(parameters)
See? Easy.
- Function_name identifies which function it is you want to use (You'd figure...). For example, the function raw_input, which will be the first function that we will use.
- Parameters are the values you pass to the function to tell it what is should do, and how to do it.. for example, if a function multiplied any given number by five, the stuff in parameters tells the function which number it should multiply by five. Put the number 70 into parameters, and the function will do 70 x 5.
Parameters and Returned Values - Communicating with Functions
Well, that's all well and good that the program can multiply a number by five, but what does it have to show for it? A warm fuzzy feeling? Your program needs to see the results of what happened, to see what 70 x 5 is, or to see if there is a problem somewhere (like you gave it a letter instead of a number). So how does a function show what is does?
Well, in effect, when a computer runs a function, it doesn't actually see the function name, but the result of what the function did. Variables do the exact same thing - the computer doesn't see the variable name, it sees the value that the variable holds. Lets call this program that multiplied any number by five, multiply(). You put the number you want multiplied in the brackets. So if you typed this:
Code Example 2 - Using a function
a = multiply(70)
The computer would actually see this:
Code Example 3 - What the computer sees
a = 350
note: don't bother typing in this code - multiply() isn't a real function, unless you create it.
The function ran itself, then returned a number to the main program, based on what parameters it was given.
Now let's try this with a real function, and see what it does. The function is called raw_input, and asks the user to type in something. It then turns it into a string of text. Try the code below:
Code Example 4 - Using raw_input
# this line makes 'a' equal to whatever you type in
a = raw_input("Type in something, and it will be repeated on screen:")
# this line prints what 'a' is now worth
print a
Say in the above program, you typed in 'hello' when it asked you to type something in. To the computer, this program would look like this:
Code Example 5 - What the computer sees
a = "hello"
print "hello"
Remember, a variable is just a stored value. To the computer, the variable 'a' doesn't look like 'a' - it looks like the value that is stored inside it. Functions are similar - to the main program (that is, the program that is running the function), they look like the value of what they give in return of running.
A Calculator Program
Lets write another program, that will act as a calculator. This time it will do something more adventerous than what we have done before. There will be a menu, that will ask you whether you want to multiply two numbers together, add two numbers together, divide one number by another, or subtract one number from another. Only problem - the raw_input function returns what you type in as a string - we want the number 1, not the letter 1 (and yes, in python, there is a difference.).
Luckily, somebody wrote the function input, which returns what you typed in, to the main program - but this time, it puts it in as a number. If you type an integer (a whole number), what comes out of input is an integer. And if you put that integer into a variable, the variable will be an integer-type variable, which means you can add and subtract, etc.
Now, lets design this calculator properly. We want a menu that is returned to every time you finish adding, subtracting, etc. In other words, to loop (HINT!!!) while (BIG HINT!!!) you tell it the program should still run.
We want it to do an option in the menu if you type in that number. That involves you typing in a number (a.k.a input) and an if loop.
Lets write it out in understandable English first:
Code Example 6 - human-language example
START PROGRAM
print opening message
while we let the program run, do this:
#Print what options you have
print Option 1 - add
print Option 2 - subtract
print Option 3 - multiply
print Option 4 - divide
print Option 5 - quit program
ask for which option is is you want
if it is option 1:
ask for first number
ask for second number
add them together
print the result onscreen
if it is option 2:
ask for first number
ask for second number
subtract one from the other
print the result onscreen
if it is option 3:
ask for first number
ask for second number
multiply!
print the result onscreen
if it is option 4:
ask for first number
ask for second number
divide one by the other
print the result onscreen
if it is option 5:
tell the loop to stop looping
Print onscreen a goodbye message
END PROGRAM
Lets put this in something that python can understand:
Code Example 7 - Python verion of menu
#calculator program
#this variable tells the loop whether it should loop or not.
# 1 means loop. anything else means don't loop.
loop = 1
#this variable holds the user's choice in the menu:
choice = 0
while loop == 1:
#print what options you have
print "Welcome to calculator.py"
print "your options are:"
print " "
print "1) Addition"
print "2) Subtraction"
print "3) Multiplication"
print "4) Division"
print "5) Quit calculator.py"
print " "
choice = input("Choose your option: ")
if choice == 1:
add1 = input("Add this: ")
add2 = input("to this: ")
print add1, "+", add2, "=", add1 + add2
elif choice == 2:
sub2 = input("Subtract this: ")
sub1 = input("from this: ")
print sub1, "-", sub2, "=", sub1 - sub2
elif choice == 3:
mul1 = input("Multiply this: ")
mul2 = input("with this: ")
print mul1, "*", mul2, "=", mul1 * mul2
elif choice == 4:
div1 = input("Divide this: ")
div2 = input("by this: ")
print div1, "/", div2, "=", div1 / div2
elif choice == 5:
loop = 0
print "Thankyou for using calculator.py!"
Wow! That is an impressive program! Paste it into python IDLE, save it as 'calculator.py' and run it. Play around with it - try all options, entering in integers (numbers without decimal points), and numbers with stuff after the decimal point (known in programming as a floating point). Try typing in text, and see how the program chucks a minor fit, and stops running (That can be dealt with, using error handling, which we can address later.)
Define Your Own Functions
Well, it is all well and good that you can use other people's functions, but what if you want to write your own functions, to save time, and maybe use them in other programs? This is where the 'def' operator comes in. (An operator is just something that tells python what to do, e.g. the '+' operator tells python to add things, the 'if' operator tells python to do something if conditions are met.)
This is how the 'def' operator works:
Code Example 8 - The def operator
def function_name(parameter_1,parameter_2):
{this is the code in the function}
{more code}
{more code}
return {value to return to the main program}
{this code isn't in the function}
{because it isn't indented}
#remember to put a colon ":" at the end
#of the line that starts with 'def'
function_name is the name of the function. You write the code that is in the function below that line, and have it indented. (We will worry about parameter_1 and parameter_2 later, for now imagine there is nothing between the parentheses.
Functions run completely independent of the main program. Remember when I said that when the computer comes to a function, it doesn't see the function, but a value, that the function returns? Here's the quote:
Functions run completely independent of the main program. Remember when I said that when the computer comes to a function, it doesn't see the function, but a value, that the function returns? Here's the quote:
To the computer, the variable 'a' doesn't look like 'a' - it looks like the value that is stored inside it. Functions are similar - to the main program (that is, the program that is running the function), they look like the value of what they give in return of running.
A function is like a miniture program that some parameters are given to - it then runs itself, and then returns a value. Your main program sees only the returned value. If that function flew to the moon and back, and then at the end had:
Code Example 9 - return
return "Hello"
then all your program would see is the string "hello", where the name of the function was. It would have no idea what else the program did.
Because it is a seperate program, a function doesn't see any of the variables that are in your main program, and your main program doesn't see any of the variables that are in a function. For example, here is a function that prints the words "hello" onscreen, and then returns the number '1234' to the main program:
Code Example 10 - using return
# Below is the function
def hello():
print "hello"
return 1234
# And here is the function being used
print hello()
Think about the last line of code above. What did it do? Type in the program (you can skip the comments), and see what it does. The output looks like this:
Code Example 11 - the output
hello
1234
So what happened?
- when 'def hello()' was run, a function called 'hello' was created
- When the line 'print hello()' was run, the function 'hello' was executed (The code inside it was run)
- The function 'hello' printed "hello" onscreen, then returned the number '1234' back to the main program
- The main program now sees the line as 'print 1234' and as a result, printed '1234
That accounts for everything that happened. remember, that the main program had NO IDEA that the words "hello" were printed onscreen. All it saw was '1234', and printed that onscreen.
Passing Parameters to functions
There is one more thing we will cover in this (monsterously huge) lesson - passing parameters to a function. Think back to how we defined functions:
Code Example 12 - Defining functions with parameters
def function_name(parameter_1,parameter_2):
{this is the code in the function}
{more code}
{more code}
return {value (e.g. text or number) to return to the main program}
Where parameter_1 and parameter_2 are (between the parentheses), you put the names of variables that you want to put the parameters into. Put as many as you need, just have them seperated by commas. When you run a function, the first value you put inside the parentheses would go into the variable where parameter_1 is. The second one (after the first comma) would go to the variable where parameter_2 is. This goes on for however many parameters there are in the function (from zero, to the sky) For example:
Code Example 13 - how parameters work
def funnyfunction(first_word,second_word,third_word):
print "The word created is: " + first_word + second_word + third_word
return first_word + second_word + third_word
When you run the function above, you would type in something like this: funnyfunction("meat","eater","man"). The first value (that is, "meat") would be put into the variable called first_word. The second value inside the brackets (that is, "eater") would be put into the variable called second_word, and so on. This is how values are passed from the main program to functions - inside the parentheses, after the function name.
A final program
Think back to that calculator program. Did it look a bit messy to you? I think it did, so lets re-write it, with functions.
To design - First we will define all the functions we are going to use with the 'def' operator (still remember what an operator is ;) ). Then we will have the main program, with all that messy code replaced with nice, neat functions. This will make it so much easier to look at again in the future.
Code Example 14 - Calculator program
# calculator program
# NO CODE IS REALLY RUN HERE, IT IS ONLY TELLING US WHAT WE WILL DO LATER
# Here we will define our functions
# this prints the main menu, and prompts for a choice
def menu():
#print what options you have
print "Welcome to calculator.py"
print "your options are:"
print " "
print "1) Addition"
print "2) Subtraction"
print "3) Multiplication"
print "4) Division"
print "5) Quit calculator.py"
print " "
return input ("Choose your option: ")
# this adds two numbers given
def add(a,b):
print a, "+", b, "=", a + b
# this subtracts two numbers given
def sub(a,b):
print b, "-", a, "=", b - a
# this multiplies two numbers given
def mul(a,b):
print a, "*", b, "=", a * b
# this divides two numbers given
def div(a,b):
print a, "/", b, "=", a / b
# NOW THE PROGRAM REALLY STARTS, AS CODE IS RUN
loop = 1
choice = 0
while loop == 1:
choice = menu()
if choice == 1:
add(input("Add this: "),input("to this: "))
elif choice == 2:
sub(input("Subtract this: "),input("from this: "))
elif choice == 3:
mul(input("Multiply this: "),input("by this: "))
elif choice == 4:
div(input("Divide this: "),input("by this: "))
elif choice == 5:
loop = 0
print "Thankyou for using calculator.py!"
# NOW THE PROGRAM REALLY FINISHES
The initial program had 34 lines of code. The new one actually had 35 lines of code! It is a little longer, but if you look at it the right way, it is actually simpler.
You defined all your functions at the top. This really isn't part of your main program - they are just lots of little programs, that you will call upon later. You could even re-use these in another program if you needed them, and didn't want to tell the computer how to add and subtract again.
If you look at the main part of the program (between the line 'loop = 1' and 'print "Thankyou for..."'), it is only 15 lines of code. That means that if you wanted to write this program differently, you would only have to write 15 or so lines, as opposed to the 34 lines you would normally have to without functions.
Tricky Ways You Can Pass Parameters
Finally, as a bit of an interlude, I will explain what the line add(input("Add this: "),input("to this: ")) means.
I wanted to fit everything onto one line, with as few variables as possible. Remember what functions look like to the main program? Whatever value they return. If the numbers you passed to the add() function were 2 and 30, the main program would see this:
Code Example 15 - The results of fancy parameter work
add(2,30)
The add program would then run, adding 2 and 30, then printing the result. The add program has no 'return' operator - it doesn't return anything to the main program. It simply adds two numbers and prints them onscreen, and the main program doesn't see anything of it.
Instead of (input("Add this: "),input("to this: ")) as the parameters for the add program you could have variables. E.g.
Code Example 16 - variables as parameters
num1 = 45
num2 = 7
add(num1,num2)
For the above, remember that the function you are passing the variables to cannot change the variables themselves - they are simply used as values. You could even put the values straight into the function:
Code Example 17 - the end result
add(45,7)
This is because the only thing the function sees are the values that are passed on as parameters. Those values are put into the variables that are mentioned when 'add' is defined (the line 'def add(a,b)' ). The function then uses those parameters to do it's job.
In short:
- The only thing functions see of the main program is the parameters that are passed to it
- the only thing the main program seens of functions is the returned value that it passes back
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