Convert multi-dimensional list to a 1D list in Python

Question

A multidimensional list like l=[[1,2],[3,4]] could be converted to a 1D one by doing sum(l,[]). Can anybody please explain how that happens?

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Answer 1

If your list nested is, as you say, "2D" (meaning that you only want to go one level down, and all 1-level-down items of nested are lists), a simple list comprehension:

flat = [x for sublist in nested for x in sublist]

is the approach I'd recommend -- much more efficient than summing would be (sum is intended for numbers -- it was just too much of a bother to somehow make it block all attempts to "sum" non-numbers... I was the original proposer and first implementer of sum in the Python standard library, so I guess I should know;-).

If you want to go down "as deep as it takes" (for deeply nested lists), recursion is the simplest way, although by eliminating the recursion you can get higher performance (at the price of higher complication).

This recipe suggests a recursive solution, a recursion elimination, and other approaches (all instructive, though none as simple as the one-liner I suggested earlier in this answer).