python - Flattening a list recursively

Question

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Answer 1

This handles both of your cases, and I think will solve the general case, without any for loops:

def flatten(S):
    if S == []:
        return S
    if isinstance(S[0], list):
        return flatten(S[0]) + flatten(S[1:])
    return S[:1] + flatten(S[1:])