As others have pointed out, that only flattens one level (which, admittedly, is a much more useful thing most of the time - arbitrarily deeply nested structures

As others have pointed out, that only flattens one level (which, admittedly, is a much more useful thing most of the time - arbitrarily deeply nested structures are relatively rare). If you do want to do this, the better method is: itertools.chain.from_iterable() That has the advantage of being lazy, as opposed to a list comprehension, and has the potential to be more optimised. It also doesn't rely on the somewhat obscure and relatively hard to read nested list comprehension syntax. Docs: https://docs.python.org/3.6/library/itertools.html#itertools.chain.from_iterable https://docs.python.org/3.6/library/itertools.html#itertools...