introduction to python

CSC462: Introduction to Artiﬁcial
Intelligence (Lab)
Department of Computer Sciences
COMSATS Institute of Information Technology
Abbottabad
Dr. Zia ur Rehman CSC462: Introduction to Artiﬁcial Intelligence (Lab)1 / 19

Introduction Python
Dr. Zia ur Rehman
COMSATS Institute of Information Technology
Abbottabad, Pakistan
Fall 2016

What is Python?
Python was created by a Dutchman named Guido van Rossum
in the early 90s.
It is now one of the most popular languages in existence.
There are two versions of Python:
Python 2.7
Python 3.5

Installing Python
There are several ways for installing Python and its associated
tools.
The easiest is to donwload and install the anaconda
distribution from:
https://www.continuum.io/downloads

Which setup to download?

Spyder IDE

Variables and Assignment
1 x = 3 # Create variable x and assign value 3 to it
2 x = x*x # Bind x to value 9
3 print(x)
Click on the variable explorer tab (below top right window) in
Spyder IDE to see the variable type at the value assigned.
Code can be type in the script window (left) or the python
console (bottom right).

Variable Types
All variables in Python are objects!
The type() function returns the variable (or literal) type.
1 y = input(’enter a number:’)
2 print (type(y))
3 print (y)
1 y = float(input(’Enter a number: ’))
2 print (type(y))
3 print (y)
4 print (y*y)

Variables in C/C++/Java vs Variables in Python
1 int x;
2 int y;
3
4
5 x=42;
6 y=42;
7
8
9 y=78;

Variables in Python
There is no declaration of variables required in Python. It’s not
even possible. If there is need of a variable, you think of a
name and start using it as a variable.
Not only the value of a variable may change during program
execution but the type as well. You can assign an integer value
to a variable, use it as an integer for a while and then assign a
string to the same variable.

Variables in Python
1 i = 42 # assgnment
2 i = i + 1 # addition operatior
3 print(i) # print the result
1 i = 42 # data type is implicitly set to
integer→
2 i = 42 + 0.11 # data type is changed to float
3 i = "fourty" # and now it will be a string
Python variables are references to objects, but the actual data is
contained in the objects

Variables in Python
1 x=42
2
3
4
5
6 y=x
7
8
9
10
11 y=78

Variables in Python
1 x = "Text" # Now x
references a string→

id Function
How can we see or prove
that x and y really
reference the same object
after the assignment y = x
the previous example?
The identity function id()
can be used for this
purpose.
Every instance (object or
variable) has an identity,
i.e. an integer which is
unique within the script or
program, i.e. other objects
have diﬀerent identities.
Type the following code in the
python console window.
1 >>> x = 42
2 >>> id(x)
3 10107136
4 >>> y = x
5 >>> id(x), id(y)
6 (10107136, 10107136)
7 >>> y = 78
8 >>> id(x), id(y)
9 (10107136, 10108288)
10 >>>

Blocks and Indentation
Other languages use { } or Begin: End: kewords to mark
code blocks. Python uses a diﬀerent principle.
Python programs get structured through indentation, i.e. code
blocks are deﬁned by their indentation.
Code indentation is used in other languages as well but in case
of Python it’s a language requirement not a matter of style.
Loops and Conditional statements end with a colon ":" - the
same is true for functions and other structures introducing
blocks.

Indentation example
1 x = int(input(’Enter an integer: ’))
2 if x%2 == 0:
3 print(’Even’)
4 else:
5 print(’Odd’)
6 if x%3 != 0:
7 print (’And not divisible by 3’)

Variable Names
The naming of variables follows the more general concept of
an identifier.
A Python identifier is a name used to identify a variable,
function, class, module or other object.
A variable name and an identifier can consist of the uppercase
letters "A" through "Z", the lowercase letters "a" through
"z", the underscore _ and, except for the first character, the
digits 0 through 9.
Python 3.x is based on Unicode. This means that variable
names and identifier names can additionally contain Unicode
characters as well.
Identifiers are unlimited in length. Case is significant.
Exceptions from the rules above are the special Python
keywords

Python Keywords I
No identiﬁer can have the same name as one of the Python
keywords, although they are obeying the above naming conventions:
1 and, as, assert, break, class, continue, def, del,
elif, else,→
2 except, False, finally, for, from, global, if, import,
in, is,→
3 lambda, None, nonlocal, not, or, pass, raise, return,
True, try,→
4 while, with, yield
There is no need to learn them by heart. You can get the list of
Python keywords in the interactive shell by using help. You type in
help() in the interactive console:

Python Keywords II
1 help()
2 help>
This will show he help prompt. Now type the following statement:
1 help> keywords

introduction to python

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