How Can I Flatten Nested List Comprehension Results in Python?

Flattening List Comprehension Results

When using list comprehensions, it's common to encounter nested lists as output, especially when working with functions that return lists. To convert these nested lists into a flat result, consider implementing the following techniques:

Nested Iterations in List Comprehensions

Python's list comprehensions support nested loops. This allows you to iterate over multiple collections and flatten the results. For instance, the following nested list comprehension converts a list of directories into a flat list of subdirectories:

<code class="python">dirs = ['dir1', 'dir2', 'dir3']
filenames = [filename for path in dirs for filename in os.listdir(path)]</code>

This comprehension iterates over dirs, accessing each directory path. Within the inner loop, it uses os.listdir to retrieve the filenames in that directory. The resulting list contains all subdirectories.

Chaining List Comprehensions

Alternatively, you can chain multiple list comprehensions to achieve flattening. This approach is suitable when you have multiple transformations to apply:

<code class="python">directories = ['dir1', 'dir2', 'dir3']
filenames = [filename for path in directories if os.path.isdir(path) for filename in os.listdir(path)]</code>

In this example, we first filter out non-directories from directories (using the if clause). Then, we apply the same os.listdir operation as in the previous example. Chaining these comprehensions flattens the results.

By employing nested iterations or chaining comprehensions, you can effectively flatten nested list comprehension results, providing a convenient way to simplify your code and maintain readability.

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