> nitpicking a bit: negative values should probably raise a proper ValueError, no?

I think they do, with this patch.  Or maybe I'm misunderstanding?

With the current form of the patch:

>>> math.factorial(10**20)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: factorial() argument outrageously large
>>> math.factorial(-10**20)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: factorial() not defined for negative values