正攻法はNullObjectパターンだろうけど、
>>655で十分なんじゃないの？と言う気もする。

class MyNullObject:
　pass

def mypop(firstArg, secondArg=MyNullObject()):
　if isinstance(secondArg, MyNullObject):
　　print "mypop(firstArg)"
　else:
　　print "mypop(firstArg, secondArg)"

>>> mypop(1)
mypop(firstArg)

>>> mypop(1, 2)
mypop(firstArg, secondArg)

>>> mypop(1, 2, 3)
Traceback (most recent call last):
File "<pyshell#24>", line 1, in <module>
mypop(1, 2, 3)
TypeError: mypop() takes at most 2 arguments (3 given)