Is Using List Comprehensions for Side Effects in Python Considered Bad Practice?

Using List Comprehensions for Side Effects: A Non-Pythonic Practice

Python's list comprehensions are powerful tools for creating new lists based on existing iterables. While they offer concise and expressive syntax, using list comprehensions solely for their side effects is considered anti-Pythonic.

The Problem

Consider a function that performs side effects such as printing to the screen, updating a GUI, or writing to a file. It returns a value, but the value is typically not of interest.

def fun_with_side_effects(x):
    ...side effects...
    return y

One might be tempted to use a list comprehension to call this function for its side effects, as follows:

[fun_with_side_effects(x) for x in y if (...conditions...)]

Note that the resulting list is not assigned to a variable.

The Anti-Pythonic Nature

Using list comprehensions in this manner is strongly discouraged for several reasons:

• Inefficiency: Creating the intermediate list can be expensive, especially if the iterable is large. The list is created but immediately discarded, resulting in wasted resources.
• Unidiomatic: Seasoned Python developers favor using explicit for loops for side-effecting operations. List comprehensions are generally used for creating new values.
• Confusion: By discarding the list, the intention of the code can become unclear to other programmers. It is preferable to make side effects explicit.

The Preferred Approach

The preferred approach is to use a for loop to iterate over the iterable and call the side-effecting function only when necessary:

for x in y:
    if (...conditions...):
        fun_with_side_effects(x)

This approach is more efficient, idiomatic, and less confusing. It is a clear indication that the function is being called primarily for its side effects.

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