Python map and filter

Basic

Python's map and filter built-ins both work with iterables, but they answer different questions. map transforms each item, while filter keeps only items that pass a test.

Transform every item with map

Pass a function and an iterable to map. The result is an iterator that yields the function result for each input item.

The function receives one item at a time and its return value becomes the corresponding item in the mapped result.

Map several iterables together

When map receives more than one iterable, the function receives one item from each iterable at the same position.

Filter items with a test

Pass a function and an iterable to filter. The function decides which original items remain.

Unlike map, filter does not replace an item with the function's return value. A truthy result keeps the original item.

Filter by truth value

When the function argument is None, filter uses the truth value of each item and removes the false ones.

Remember that both results are iterators

Both built-ins return iterators. Iterating over the result consumes it, so a second pass over the same result produces no more values.

Choose the operation that matches the intent

Use map when every item should become a new value. Use filter when some original items should be kept and others discarded.