Where is the default parameter in Python function

Question

I think many people have seen the python\'s function which receives default parameters. For example:

def foo(a=[]):
    a.append(3)
    return a

Answer 1

It's attached to the function object, see foo.func_defaults:

>>> foo()
>>> foo.func_defaults
([3],)
>>> foo()
>>> foo.func_defaults
([3, 3],)

In case if you want to get the mapping of a onto [], you can access foo.func_code:

defaults = foo.func_defaults
# the args are locals in the beginning:
args = foo.func_code.co_varnames[:foo.func_code.co_argcount] 
def_args = args[-len(defaults):]  # the args with defaults are in the end
print dict(zip(def_args, defaults)) # {'a': []}

(But, apparently, the yak's version is better.)