.. Objectives

.. ----------

.. Prerequisites

.. -------------

.. Author              : Madhu

   Internal Reviewer   : 

   External Reviewer   :

   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

Script

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In this tutorial, we will learn the basic conditional constructs

available in Python. We learn the if/else, if/elif/else and ternary

conditional constructs available in Python. 

{{{ Shift to terminal and start ipython }}}

To begin with let us start ipython, by typing::

  ipython

on the terminal

Whenever we have two possible states that can occur depending on a

whether a certain condition we can use if/else construct in

  a = 5

In such a case we can write the if/else block as::

  if a % 2 == 0:

      print "Even"

  else:

      print "Odd"

  a = -3

  if a > 0:

      print "positive"

  elif a < 0:

      print "negative"

  else:

      print "zero"

optional. We can have a single if statement or just if/elif statements

without having else block at all. Also, there can be any number of

elif's within an if/elif/else ladder. For example

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  if user == 'admin':

      # Do admin operations

  elif user == 'moderator':

      # Do moderator operations

  elif user == 'client':

      # Do customer operations

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is completely valid. Note that there are multiple elif blocks and there

is no else block.

In addition to these conditional statements, Python provides a very

convenient ternary conditional operator. Let us take the following

example where we read the marks data from a data file which is

obtained as a string as we read a file. The marks can be in the range

of 0 to 100 or 'AA' if the student is absent. In such a case to obtain

the marks as an integer we can use the ternary conditional

operator. Let us say the string score is stored in score_str

variable::

  score_str = 'AA'

Now let us use the ternary conditional operator::

  score = int(score_str) if score_str != 'AA' else 0

This is just the if/else statement block which written in a more

convenient form and is very helpful when we have only one statement

for each block. This conditional statement effectively means as we

would have exactly specified in the English language which will be

like score is integer of score_str is score_str is not 'AA' otherwise

it is 0. This means that we make the scores of the students who were

absent for the exam 0.

otherwise. In such cases "pass" statement comes very handy::

  a = raw_input("Enter 'c' to calculate and exit, 'd' to display the existing

  results exit and 'x' to exit and any other key to continue: ")

  if a == 'c':

     # Calculate the marks and exit

  elif a == 'd':

     # Display the results and exit

  elif a == 'x':

     # Exit the program

  else:

     pass

In this case "pass" statement acts as a place holder for the block of

code. It is equivalent to a null operation. It literally does

implementation for a particular block of code is not known yet but has

to be filled up later.

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This brings us to the end of the tutorial session on conditional

statements in Python. In this tutorial session we learnt

  * What are conditional statements

  * if/else statement

  * if/elif/else statement

  * Ternary conditional statement - C if X else Y

  * and the "pass" statement

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.

Thank you!