What's the Most Efficient Way to Flatten a List of Lists in Python?

Flattening Lists of Lists

Flattening a list of lists involves converting a nested structure into a single-level list. In Python, this can be achieved using several methods.

Nested List Comprehension

The most efficient method is a nested list comprehension:

flat_list = [x for xs in xss for x in xs]

This expression iterates over each inner list and appends each element to the resulting flat list.

Using the Operator

You can also flatten lists using the operator:

flat_list = []
for xs in xss:
    flat_list = flat_list + xs

This method is marginally slower than the list comprehension due to repeated list concatenation.

Using sum with an Empty List

Another approach uses the sum function with an empty list:

flat_list = sum(xss, [])

This method is less efficient than the list comprehension, especially for large lists.

Using functools.reduce

Finally, you can use the functools.reduce function:

from functools import reduce
flat_list = reduce(lambda xs, ys: xs + ys, xss)

Again, this method is slower than the list comprehension.

Performance tests using the timeit module show that the list comprehension is the fastest method.

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