Prev: - 'while' loop | Next: Break, continue, pass

'If' control¶

if¶

The 'if' command evaluates the truth value of a conditional expression (or 'condition') and changes the program flow depending on the value of the condition (True or False)
The general form of the Python 'if' command is as follows:

if condition-1:  
    block-1  
elif condition-2:    
    block-2  
...........................       
elif condition-N:    
    block-N  
else:    
    block_else

If condition-1 is True then block-1 is executed (and the if command is exited)
If, however, condition-1 is False then the conditions in the elif conditions are tested. If one of them is True then the respective block is executed (and the if structure command is exited)
If no elif exists or all elif conditions are False then the 'else' block is executed

Simple 'if' examples¶

# Python allows concise forms of composite conditions
x = int(input())
if 0 < x < 10:          # instead of:  0<x and x<10
    print('x in the (0,10) interval')
else:
    print('x out of the (0,10) interval')

5
x in the (0,10) interval

# When implementing multiple if branches: use elif - avoid if-else nesting    

grade = int(input())
if 18.5 <= grade <= 20:
    perform = 'Excellent'
elif 15 <= grade < 18.5:
    perform = 'Very Good'
elif 12.5 <= grade < 15:
    perform = 'Good'
elif 10 <= grade < 12.5:
    perform = 'Accept'
else:
    perform = 'Fail'

print(perform)

15
Very Good

import random 

a = random.randint(1,3)

if a == 2:
    print('Equal')
elif a < 2:
    print('Less than 2')
else:
    print('Greater than 2')

print('End of if control, a =',a)

Less than 2
End of if control, a = 1

# Note the use of docstring enclosed in triple quotes
menu = '''
Please make a choice:
        1. spam
        2. chorizo
        3. salchichón
        4. jamon iberico de belotta
'''
print(menu)
c = input('Your choice: ')

if c=='1':
    print('spam: €0.25')
elif c=='2':
    print('chorizo: €2.45')
elif c=='3':
    print('salchichón = €1.99')
elif c=='4':
    print('jamon iberico de belotta = €100.10')
else:
    print('Bad choice')

Please make a choice:
        1. spam
        2. chorizo
        3. salchichón
        4. jamon iberico de belotta

Your choice: 1
spam: €0.25

Conditional expression¶

Python has also conditional expressions in the form:

  "True_expression if Condition else False_expression"

x=False
a=1 if x else 2
a

2

a = 'Alpha'
b='1'
print(a) if b else 'X'

Alpha

Conditional expressions will be very useful in constructing lists with the list comprehension method as we shall see later on

Copyright¶

. Free learning material
. See full copyright and disclaimer notice