introduction to python

CSC462: Introduction to ArtificialIntelligence (Lab)

Department of Computer SciencesCOMSATS 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 TechnologyAbbottabad, Pakistan

Fall 2016


What is Python?

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

Python 2.7Python 3.5


Installing Python

There are several ways for installing Python and its associatedtools.The easiest is to donwload and install the anacondadistribution from:https://www.continuum.io/downloads


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) inSpyder IDE to see the variable type at the value assigned.Code can be type in the script window (left) or the pythonconsole (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 noteven possible. If there is need of a variable, you think of aname and start using it as a variable.Not only the value of a variable may change during programexecution but the type as well. You can assign an integer valueto a variable, use it as an integer for a while and then assign astring 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 iscontained 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 provethat x and y reallyreference the same objectafter the assignment y = xthe previous example?The identity function id()can be used for thispurpose.Every instance (object orvariable) has an identity,i.e. an integer which isunique within the script orprogram, i.e. other objectshave different identities.

Type the following code in thepython 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 markcode blocks. Python uses a different principle.Python programs get structured through indentation, i.e. codeblocks are defined by their indentation.Code indentation is used in other languages as well but in caseof Python it’s a language requirement not a matter of style.Loops and Conditional statements end with a colon ":" - thesame is true for functions and other structures introducingblocks.


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 ofan 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 uppercaseletters "A" through "Z", the lowercase letters "a" through"z", the underscore _ and, except for the first character, thedigits 0 through 9.Python 3.x is based on Unicode. This means that variablenames and identifier names can additionally contain Unicodecharacters as well.Identifiers are unlimited in length. Case is significant.Exceptions from the rules above are the special Pythonkeywords


Python Keywords I

No identifier can have the same name as one of the Pythonkeywords, 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 ofPython keywords in the interactive shell by using help. You type inhelp() 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


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