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:
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    # 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)

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    Output: 
    10
    
  2. Direct Method by using tuples, Dictionary and lambda
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    # 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]())

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    Output:
    10
    10
    10
    
  3. Ternary operator can be written as nested if-else:
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    # 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")

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    Above approach can be written as:

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    # 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")

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    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 :

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# 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)

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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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