i thought this code be solved nicely with a decorator... it needs some more work, but it would make a good cookbook recipe: .>>> from copy import deepcopy. .>>> .>>> def defaults(**kwdefs): ... def deco(func): ... def wrapper(*args, **kwargs): ... for k,v in kwdefs.iteritems(): ... if k not in kwargs: ... kwargs[k] = deepcopy(v) ... return func(*args, **kwargs) ... return wrapper ... return deco ... .>>> @defaults(x = []) ... def foo(a, x): ... x.append(a) ... print x ... .>>> foo(5) [5] .>>> foo(5) [5] .>>> foo(5) [5] maybe it should be done by copying func_defaults... then it could be written as @copydefaults def f(a, b = 5, c = []): ... -tomer