Python¶
Python is an general purpose object oriented programming language that was created by Guido Van Rossum. The language has many implementations e.g. CPython, Jython, Pypy, ...etc. We focus on the CPython implementaion in this course, and feel free to check the rest of them out.
Python is widly used programing language, therefore it is being taught by many schools as an introductory programming language in computer science classes. In this course we will try to teach you how to use it.
From now on when we use the word python we mean the CPython implementation of the Python programming language.
Now to start a terminal session using ALT+CTRL+T and launch the python
REPL.
Once you started the interpreter you should see the following output
Python 2.7.5 (default, Mar 9 2014, 22:15:05)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
This is the python prompt it, and it shows useful information about the current python version and the compiler that python was compiled with.
The >>> is the begining of the REPL and anything you type and hit Enter
it will evaluate on the second line and print out the result. Once it’s done
executing it will display the same prompt again waiting for the next command.
Now we will learn about the useful operations that python offers in the command line.
Integer Operations¶
>>> 3 + 7 # Addition
10
>>> 10 - 9 # Subtraction
1
>>> 9 - 100 # Handles negative numbers
-91
>>> 5 * 6 # Multiplications
30
>>> 15 / 5 # division
3
>>> 14 / 3 # integer division
4
>>> 14 // 3 # forcing integer division
4
>>> 5 ** 7 # Power operator
78125
>>> 10 % 4 # modules operations
2
>>> 1 << 4 # Left bitshift
16
>>> 16 >> 2 # Right bitshift
4
>>> 3 & 2 # Bit And operator
2
>>> 1 | 4 # Bit Or operator
5
>>> ~ 1 # 2's complement
-2
>>> 1 ^ 10 # XOr operator
11
Complex numbers¶
Python supports complex numbers by default. To use or create a complex number just append a j to a number.
>>> 4 + 6j
(4+6j)
Float point operations¶
Just like integers, Python supports float point operations
>>> 0.5 + 6.9
7.4
>>> 7 / 3.0
2.3333333333333335
>>> 7 / 2.0
3.5
>>> 4 * 8.4
33.6
>>> .5 - 8.4
-7.9
Boolean operations¶
>>> True
True
>>> False
False
>>> True or False
True
>>> True and True
True
>>> True and False
False
>>> False or False
False
>>> not False or False
True
Variables¶
To create a variable in Python, you have to use the assignment operator which is =. The has to start with latin alphabets or _
Note
This was changed in Python 3 where you can use any unicode charector so you can do the following س = 10 and Python would accept that.
>>> x = 3
>>> _r = 100
>>> x
3
>>> _r
100
>>> y = x + _r
>>> y
103
Strings¶
Python would treat any thing inside ", ' or ''' as a valid string.
>>> "This is a string"
'This is a string'
>>> 'Single quotes can be used like "'
'Single quotes can be used like "'
>>> x = """
... This is a multiline string
... The idea behind it is that you don't
... have to escape charactors
... like ' or "
... or even a new line \n
... """
>>> x
'\nThis is a multiline string\nThe idea behind it is that you don\'t \nhave to escape charactors\nlike \' or "\nor even a new line \n\n'
>>> print x
This is a multiline string
The idea behind it is that you don't
have to escape charactors
like ' or "
or even a new line
>>>
Print¶
Python has a keyword to print to the screen just like printf or cout in different languages. Print is very easy to use and it appends \n at the end of the output unless you append , where that behavior will be over written.
>>> y = "This is a line"
>>> g = "This is another line"
>>> print y
This is a line
>>> print g
This is another line
>>> print y,g
This is a line This is another line
>>> print y,g,
This is a line This is another line
Functions¶
To define a function is Python, you have to use the keyword def and then the name of the function. The name of the function can be any valid identifier. After the name of the function you have to add () to denotes function can be called without arguments or (x,y,z) to denotes a function that takes three arguments. Then, to start implementing the body of the function you have to end the function declaration with :. The body of the function must be indented by four spaces.
Note
The word must is from PEP 8 to stay consistent across files. The previous paragraph assumes mulitline functions without ;
>>> def main():
... print "Hello world"
...
>>>
>>> main()
Hello world
>>> def another_main(x,y,z):
... return (x/y)**z
...
>>>
>>>
>>> another_main(4,6,2)
0
>>> another_main(40,6,2)
36
>>> another_main(40,6.0,2)
44.44444444444445
Note
Because the language is dynamiclly typed I didn’t have to specify the type for x, y or z and I can pass anything as x, y and z. So I passed integers or floats.