List flattening

flatten.py

# Python does not have tail recursion optimization, 
# so this function can't handle too deep data structures.
# This piece of code was written as a practicing task.

def flatten(l):
    nested = None
    for i, e in enumerate(l):
        if isinstance(e, list):
            nested = e; break
    if nested is not None:
        return flatten(l[:i] + nested + l[i+1:])
    else: return l

def flatten2(l):
    if not isinstance(l, list): return [l]
    if not l: return l
    return flatten(l[0]) + flatten(l[1:])