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 python ternary operator

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Ternary operators also known as conditional expressions are operators that evaluate something based on a condition being true or false. It was added to Python in version 2.5.

Python - Work flow of ternary operator

It simply allows to test a condition in a single line replacing the multiline if-else making the code compact.

Syntax :

[on_true] if [expression] else [on_false]
  1. Simple Method to use ternary operator:

python - Sample - python code :

# Program to demonstrate conditional operator
a, b = 10, 20
 
# Copy value of a in min if a < b else copy b
min = a if a < b else b
 
print(min)

python tutorial - Output :

10
  1. Direct Method by using tuples, Dictionary and lambda

python - Sample - python code :

# Python program to demonstrate ternary operator
a, b = 10, 20
 
# Use tuple for selecting an item
print( (b, a) [a < b] )
 
# Use Dictionary for selecting an item
print({True: a, False: b} [a < b])
 
# lamda is more efficient than above two methods
# because in lambda  we are assure that
# only one expression will be evaluated unlike in
# tuple and Dictionary
print((lambda: b, lambda: a)[a < b]())

python tutorial - Output :

10
10
10
  1. Ternary operator can be written as nested if-else:

python - Sample - python code :

# Python program to demonstrate nested ternary operator
a, b = 10, 20
 
print ("Both a and b are equal" if a == b else "a is greater than b"
        if a > b else "b is greater than a")

Above approach can be written as:

python - Sample - python code :

# Python program to demonstrate nested ternary operator
a, b = 10, 20
 
if a != b:
    if a > b:
        print("a is greater than b")
    else:
        print("b is greater than a")
else:
    print("Both a and b are equal")

python tutorial - Output :

b is greater than a


Ternary Operator python

Important Points:

  • First the given condition is evaluated (a < b), then either a or b is returned based on the Boolean value returned by the condition
  • Order of the arguments in the operator is different from other languages like C/C++ (See C/C++ ternary operators).
  • Conditional expressions have the lowest priority amongst all Python operations.
Ternary Operator condition block python

Method used prior to 2.5 when ternary operator was not present
In an expression like the one given below , the interpreter checks for the expression if this is true then on_true is evaluated, else the on_false is evaluated.

Syntax :

'''When condition becomes true, expression [on_false]
   is not executed and value of "True and [on_true]"
   is returned.  Else value of "False or [on_false]"
   is returned.
   Note that "True and x" is equal to x. 
   And "False or x" is equal to x. '''
[expression] and [on_true] or [on_false]

Example :

# Program to demonstrate conditional operator
a, b = 10, 20
 
# If a is less than b, then a is assigned
# else b is assigned (Note : it doesn't 
# work if a is 0.
min = a < b and a or b
 
print(min)

python tutorial - Output :

10

Note : The only drawback of this method is that on_true must not be zero or False. If this happens on_false will be evaluated always. The reason for that is if expression is true, the interpreter will check for the on_true, if that will be zero or false, that will force the interpreter to check for on_false to give the final result of whole expression.


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