How Does Nested List Comprehension Flatten a List in Python?

Nested List Comprehension: Breaking Down the Syntax

In Python, list comprehension is a concise way to generate a new list based on an existing one. When it comes to nested list comprehensions, however, the syntax can get a bit confusing.

Consider the following snippet:

a = [[1,2],[3,4],[5,6]]
b = [x for xs in a for x in xs]

This code transforms the nested list a into a flattened list b containing all the elements from a. To understand how this works, let's break down the syntax:

[x for xs in a for x in xs]

This is a two-level nested comprehension. It's essentially saying:

# Loop over the outer list
for xs in a:
    # Loop over the inner list
    for x in xs:
        # Add x to the new list b
        b.append(x)

In other words, it first iterates over the outer list a, assigns each sublist to the variable xs, and then iterates over the sublist, assigning each element to the variable x. For each element x, it adds it to the new list b.

This unrolling process follows the order in which the loops are written: the outermost loop varies the slowest, followed by the inner loops. This is the "Right One" principle mentioned in PEP202, which says that comprehensions should be written with the fastest-varying index on the innermost loop.

By understanding this principle, you can confidently decipher even complex nested list comprehensions.

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