In Python 3, map returns a iterator, not a list: In [1]: sq = lambda x: x * x In [2]: map(sq, [1,2,3,4]) Out[2]: <builtins.map at 0x2135050> In [

In Python 3, map returns a iterator, not a list: In [1]: sq = lambda x: x * x In [2]: map(sq, [1,2,3,4]) Out[2]: <builtins.map at 0x2135050> In [3]: list(_) Out[3]: [1, 4, 9, 16]