Python List Comprehension If Else: Choose Values or Filter Rows

Start with a small, explicit task

Turn fictional check results into display labels. In the first result, keep three positions and mark failed checks as REVIEW. In the second, retain only the successful check positions. Predict both the labels and the result lengths before running the example.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_list_comprehension_if_else.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

checks = [True, False, True]
displayed = ["READY" if passed else "REVIEW" for passed in checks]
retained = ["READY" for passed in checks if passed]
assert displayed == ["READY", "REVIEW", "READY"]
assert retained == ["READY", "READY"]
assert len(displayed) == 3 and len(retained) == 2
assert ["READY" if value else "REVIEW" for value in []] == []
print(displayed)
print(retained)
print(len(displayed), len(retained))

Explain the result

The conditional expression produces exactly one label for each of the three booleans. False does not remove the middle position; it selects REVIEW. The filter version evaluates its trailing condition before producing the element, so the false input contributes nothing. Both programs are valid, but only the first preserves positional alignment with the original checks.

The empty-input assertion confirms that a conditional expression does not create a fallback item when there are no inputs. The else branch is a per-item choice, not an alternative for the entire comprehension. If you need a special message for an empty collection, handle that separately.

Avoid these mistakes

A filter clause cannot be followed by else in the same position. Put the complete conditional expression before for when selecting between two values. Keep a conventional loop when the transformation requires several statements or complicated branching; shortening code is not useful if it hides which inputs survive.