Setdefault bypasses __setitem__

[Duncan Booth]

Diez B. Roggisch wrote:

> So if setdefault
> was implemented as
> 
> def setdefault(self, v):
>      self["SOME_DEFAULT_KEY_NAME"] = v

if setdefault was implemented that way then all current uses of setdefault 
would throw an exception.

setdefault takes *three* parameters: self, key, value. Once you include the 
key parameter your entire argument implodes.