How to Flatten Nested Lists from List Comprehensions in Python?

Flattening Results of List Comprehension

In Python, using a list comprehension to apply a function to each element of a list can result in nested lists. For instance, considering a list A and a function f that transforms each element of A into a list, the list comprehension [f(a) for a in A] produces a list of lists.

To obtain a flattened list, not unlike functional programming's 'flatmap' or .NET's 'SelectMany' operators, nesting can be introduced into the list comprehension:

<code class="python">[filename for path in dirs for filename in os.listdir(path)]</code>

This is functionally equivalent to the following code that uses multiple nested loops:

<code class="python">filenames = []
for path in dirs:
    for filename in os.listdir(path):
        filenames.append(filename)</code>

Adopting this approach ensures that a flat list is produced, where elements are obtained by first iterating over the outer list dirs and then the inner list returned by the function f applied to each element in dirs.

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