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Python Conditional Expression

Conditional Expressions (also known as Ternary Operators) are used to write an if statement in one line.

Conditional Expressions​

if-else Conditional Expression​

In Python, conditional expression syntax is as follows:

value_if_true if condition else value_if_false

where:

  • The condition is evaluated first.
  • If condition is True, value_if_true is evaluated and its value is returned
  • If condition is False, value_if_false is evaluated and its value is returned

For example:

a = 1
result = 'even' if a % 2 == 0 else 'odd'
print(result) # odd

a = 2
result = 'even' if a % 2 == 0 else 'odd'
print(result) # even

The equivalent extended form is:

if a % 2 == 0:
    result = 'even'
else:
    result = 'odd'
note

An expression that does not return a value (i.e., an expression that returns None) is also acceptable.

a = 1
print('even') if a % 2 == 0 else print('odd') # odd
note

You can also concatenate multiple conditional expressions with and and or.

a = -2
result = 'negative and even' if a < 0 and a % 2 == 0 else 'positive or odd'
print(result) # negative and even

if-elif-else by conditional expressions​

value_if_true if condition1 else value_if_true2 if condition2 else value_else

For example:

a = 2
result = 'negative' if a < 0 else 'positive' if a > 0 else 'zero'
print(result) # positive

Conditional Expressions with Tuple Notation​

An alternative but very little-used form of Conditional Expressions involves tuples.

(if_test_is_false, if_test_is_true)[condition]

For example:

num = 10
result = ("Even", "Odd")[num % 2]
print("The number is ", result) # The number is Even
note

If the condition is true, the value at position 1 of the tuple is returned, otherwise the value at position 0 is returned.

warning

It is easy to confuse where to put the true value and where to put the false value in the tuple. Also, both elements of the tuple are evaluated, whereas the ternary if-else operator does not, and this can lead to errors.

condition = True
print(2 if condition else 1/0) # 2

print((1/0, 2)[condition]) 
#ZeroDivisionError is raised

Common cases​

Conditional Expressions in List Comprehensions​

Using conditional expressions in list comprehension, you can apply operations to list elements based on the condition.

my_list = ['even' if i % 2 == 0 else 'odd' for i in range(10)]
print(my_list)
# ['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']
l = [i * 2 if i % 2 == 0 else i for i in range(10)]
print(l)
# [0, 1, 4, 3, 8, 5, 12, 7, 16, 9]

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