How to Correctly Use if/else in Python List Comprehensions?

List Comprehension with if/else Conditional Expression

In Python, list comprehensions provide an elegant way to transform iterable elements into a new list. However, it can be tricky to translate traditional for-loops containing if/else statements into list comprehensions.

Problem Statement:

Consider the following for-loop:

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

The goal is to have '' if x is None and f(x) otherwise using a list comprehension. An attempt at writing this as:

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

results in a SyntaxError. What is the correct syntax?

Solution:

The solution lies in the ordering of the elements within the list comprehension. The correct syntax is:

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

Generalization:

In general, for list comprehensions with if/else conditional expressions, the syntax is:

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

For list comprehensions with if conditions only (excluding the else clause), the syntax simplifies to:

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

It's important to distinguish between list comprehensions that filter elements based on a condition and conditional expressions that evaluate to a specified value based on a condition.

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