Ternary Operator in Python

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.
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:
    # 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

  2. Direct Method by using tuples, Dictionary and lambda
    # 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]())

  3. Ternary operator can be written as nested if-else:
    # 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 program to demonstrate nested ternary operator
    a, b = 10, 20
    if a != b:
        if a > b:
            print("a is greater than b")
            print("b is greater than a")
        print("Both a and b are equal")

    Output: b is greater than a

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.

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


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.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

This article is attributed to GeeksforGeeks.org

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