class method generates "TypeError: ... got multiple values for keyword argument ..."
If I define a class method with a keyword argument thus:
class foo(object): def foodo(thing=None, thong='not underwear'): print thing if thing else "nothing" print 'a thong is',thong
calling the method generates a
myfoo = foo() myfoo.foodo(thing="something") ... TypeError: foodo() got multiple values for keyword argument 'thing'
What's going on?
The problem is that the first argument passed to class methods in python is always a copy of the class instance on which the method is called, typically labelled
self. If the class is declared thus:
class foo(object): def foodo(self, thing=None, thong='not underwear'): print thing if thing else "nothing" print 'a thong is',thong
it behaves as expected.
self as the first parameter, when
myfoo.foodo(thing="something") is executed, the
foodo method is called with arguments
(myfoo, thing="something"). The instance
myfoo is then assigned to
thing is the first declared parameter), but python also attempts to assign
thing, hence the Exception.
To demonstrate, try running this with the original code:
myfoo.foodo("something") print print myfoo
You'll output like:
<__main__.foo object at 0x321c290> a thong is something <__main__.foo object at 0x321c290>
You can see that 'thing' has been assigned a reference to the instance 'myfoo' of the class 'foo'. This section of the docs explains how function arguments work a bit more.