Why are functions atomic?

[Michael]

Why are functions atomic?  (I.e. they are not copied.)

For example, I would like to make a copy of a function so I can change
the default values:

>>> from copy import copy
>>> f = lambda x: x
>>> f.func_defaults = (1,)
>>> g = copy(f)
>>> g.func_defaults = (2,)
>>> f(),g()
(2, 2)

I would like the following behaviour:

>>> f(),g()
(1,2)

I know I could use a 'functor' defining __call__ and using member
variables, but this is more complicated and quite a bit slower.  (I
also know that I can use new.function to create a new copy, but I
would like to know the rational behind the decision to make functions
atomic before I shoot myself in the foot;-)

Thanks,
Michael.