How to Flatten Nested List Comprehensions in Python?

Flattening a List Comprehension to Obtain a Single-Leveled Result

When faced with a nested data structure, programmers often aim to flatten it into a single level. This arises while using list comprehensions, where each element might output a list, leading to a list of lists. The question at hand tackles this issue, seeking a solution to obtain a flattened list.

Nested Iterations for Flattening

Python's list comprehensions offer a concise syntax for intricate data transformations. To achieve flattening, one can employ nested iterations within a single list comprehension. The following code demonstrates this approach:

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

In this example, 'dirs' is a list of directories, and for each directory, 'os.listdir(path)' returns a list of subdirectories. The nested comprehension iterates over both lists, accumulating the subdirectories into a flattened result.

Functional Equivalence

The nested list comprehension can be represented as a series of nested loops:

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

Both approaches achieve the same result: a flattened list of subdirectories. The list comprehension offers a more compact and readable syntax, especially for complex transformations involving multiple iterations.

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