Flattening a list recursively [duplicate]

Answer 1

li=[[1,[[2]],[[[3]]]],[['4'],{5:5}]]
flatten=lambda l: sum(map(flatten,l),[]) if isinstance(l,list) else [l]
print flatten(li)

Answer 2

Here's a possible solution without any loops or list comprehensions, just using recursion:

def flatten(test_list):
    if isinstance(test_list, list):
        if len(test_list) == 0:
            return []
        first, rest = test_list[0], test_list[1:]
        return flatten(first) + flatten(rest)
    else:
        return [test_list]

Answer 3

Well, if you want it a lisp way, let's have it.

atom = lambda x: not isinstance(x, list)
nil  = lambda x: not x
car  = lambda x: x[0]
cdr  = lambda x: x[1:]
cons = lambda x, y: x + y

flatten = lambda x: [x] if atom(x) else x if nil(x) else cons(*map(flatten, [car(x), cdr(x)]))

Answer 4

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:])