Скачать или смотреть Using map() with a List Comprehension in Python: How and Why

Learn how to use Python's `map()` function with list comprehensions, why it sometimes seems to do nothing, and how to properly consume its results.
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Introduction

Python’s map() function applies a given function to each item of an iterable. A common question is whether the iterable argument to map() can itself be a list comprehension. The short answer: yes. However, sometimes the result might seem puzzling if you don’t consume the map object.

The Problem: Why Does map() Seem to Do Nothing?

Consider this code snippet:

[[See Video to Reveal this Text or Code Snippet]]

You might expect result to contain even numbers [0, 2, 4, 6, 8], but it remains empty.

Why?

map() returns a lazy iterator—it doesn't execute the function calls immediately.

If you never iterate over this map object, the function (here, result.append) never runs.

How to Fix It

Option 1: Iterate Over the Map

Force evaluation by iterating through the map either with a loop or other consuming construct:

[[See Video to Reveal this Text or Code Snippet]]

The loop triggers the execution of result.append for each element.

Option 2: Convert to a List

Wrap the map() call with list() to immediately evaluate:

[[See Video to Reveal this Text or Code Snippet]]

Option 3: Use a List Comprehension Instead

More Pythonic and clearer to use a list comprehension:

[[See Video to Reveal this Text or Code Snippet]]

Or if you want to transform or process elements:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

The second argument to map() can be any iterable, including a list comprehension.

map() returns an iterator and is lazy; you must consume it to execute the function calls.

Use list(map(...)) or a for-loop to force evaluation.

Often, list comprehensions offer a clearer and more Pythonic solution.



Mastering these nuances helps write clean, efficient Python code when processing iterables.