How can I write an if-then-else statement in Python in one line?

General Discussions
#1

How can I write an if-then-else statement in Python in one line? For example, I want a one-line version of the following:

if count == N:
    count = 0
else:
    count = N + 1

In Objective-C, I would write this as:

count = count == N ? 0 : count + 1;
#2

In Python, you can use the ternary operator expression syntax to write an if-then-else statement in one line:

value_when_true if condition else value_when_false

For example:

'Yes' if fruit == 'Apple' else 'No'

This assigns 'Yes' to the variable if fruit is 'Apple', otherwise it assigns 'No'.

With assignment:

fruit = 'Apple'
isApple = True if fruit == 'Apple' else False

This assigns True to isApple if fruit is 'Apple', otherwise it assigns False.

Contrasting with the if syntax:

fruit = 'Apple'
isApple = False
if fruit == 'Apple':
    isApple = True

This accomplishes the same thing as the ternary operator expression but in a more verbose way.

#3

Adding to Madhuri ,

In Python, the shorthand syntax for the ternary operator is valid for actual values, including constants and functions. For example, you can use it like this:

'Yes' if fruit == 'Apple' else print('No Apple')

However, you cannot use it with keywords like raise:

'Yes' if fruit == 'Apple' else raise Exception('No Apple')

This will result in a syntax error.

#4

General ternary syntax in Python:

value_true if <test> else value_false

Another way to achieve the same result is using list indexing:

[value_false, value_true][<test>]

For example:

count = [0, N + 1][count == N]

Note that this evaluates both branches before choosing one. If you want to only evaluate the chosen branch, you can use lambda functions:

[lambda: value_false, lambda: value_true][<test>]()

For example:

count = [lambda: 0, lambda: N + 1][count == N]()