Flatten list in Python

flatten.py

def flatten(l):
    if isinstance(l, list):
        out = []
        out = flatten(l.pop()) + out
        if l:
            out = flatten(l) + out
        return out
    return [l]

# Example 
l = [3, 1, [4, 1], [[5], [9]], [2, [6, 5]]] 
print(flatten(l))