Don't use mutable types for default parameters

Using mutable types for default parameters can lead to buggy code.

Don't do this:
def foo(x=dict()):
    pass

Do this:
def foo(x=None):
    if x is None:
        x = dict()
    pass

For more info see Default Parameters: http://docs.python.org/2/reference/compound_stmts.html#function-definitions