List Comprehension with if/else: Syntax and Usage

When working with list comprehensions in Python, it's encountered to incorporate if/else logic to handle conditional operations. This article addresses the correct syntax for such scenarios.

A common task is to create a list based on a source sequence, with the following for-loop containing an if/else construct:

results = []
for x in xs:
    results.append(f(x) if x is not None else '')

This code appends '' to the result list if x is None, and otherwise calculates f(x). The desired result can also be achieved using a list comprehension, but with a necessary adjustment in the syntax:

results = [f(x) if x is not None else '' for x in xs]

The key difference lies in the order of the if and else clauses within the list comprehension. The correct syntax requires placing the if clause before the for portion, followed by the else clause.

In general, the syntax for a list comprehension with an if/else condition is:

[f(x) if condition else g(x) for x in sequence]

For list comprehensions involving only if conditions for filtering elements from the source sequence, the syntax is:

[f(x) for x in sequence if condition]

Note that conditional expressions, used in the if/else syntax of list comprehensions, are distinct from the if construct employed for filtering. Conditional expressions allow for choosing between two expressions based on a condition, similar to the ternary operator ?: found in other languages. For instance:

value = 123
print(value, 'is', 'even' if value % 2 == 0 else 'odd')

This example prints whether the value is even or odd using a conditional expression.