# Flattening List Comprehensions in Python

Python has list comprehensions, syntactic sugar for building lists from an expression.

>>> [2 * i for i in (2, 3, 5, 7, 11)] [4, 6, 10, 14, 22]

This doesn’t work so well when the comprehension expression is itself a list: you end up with a list of lists.

>>> def gen(): ... for l in [['a', 'b'], ['c'], ['d', 'e', 'f']]: ... yield l ... >>> [l for l in gen()] [['a', 'b'], ['c'], ['d', 'e', 'f']]

This is ugly. Here’s one way to build a flattened list, but it’s less elegant than the comprehension.

>>> x = [] >>> for l in gen(): ... x.extend(l) ... >>> x ['a', 'b', 'c', 'd', 'e', 'f']

It took me a while to find a readable list comprehension,
with a little help from Google.
Use sum() on the outer list and prime it with an empty list, `[]`.
Python will concatenate the inner lists, producing a flattened list.

>>> sum([l for l in gen()], []) ['a', 'b', 'c', 'd', 'e', 'f']

Alternatively, you can use itertools.chain().

>>> import itertools >>> list(itertools.chain(*gen())) ['a', 'b', 'c', 'd', 'e', 'f']

That might be slightly more efficient,
though I find the `sum()` to be a little more readable.

>>> import itertools >>> list(itertools.chain(*gen())) ['a', 'b', 'c', 'd', 'e', 'f']

That might be slightly more efficient,
though I find the `sum()` to be a little more readable.

**Edit**: I forgot about nested comprehensions

>>> [inner ... for outer in gen() ... for inner in outer] ['a', 'b', 'c', 'd', 'e', 'f']

Somewhat cryptic on one line however:

>>> [j for i in gen() for j in i] ['a', 'b', 'c', 'd', 'e', 'f']

**Update**: The nested comprehension became one of my most popular StackOverflow answers.