Setdefault bypasses __setitem__

[Diez B. Roggisch]

Ron Garret wrote:
> Is this a bug or a feature?
> 
> class mydict(dict):
>    def __setitem__(self, key, val):
>      print 'foo'
>      dict.__setitem__(self, key, val)
> 
> 
>>>>d=mydict()
>>>>d[1]=2
> 
> foo
> 
>>>>d.setdefault(2,3)


Feature. If it wouldn't bypass __setitem__, how exactly would you make a 
default-item? Using __setitem__ implies a key. So if setdefault
was implemented as

def setdefault(self, v):
     self["SOME_DEFAULT_KEY_NAME"] = v

and later on one writes e.g. a HTML-page with a form input field named 
"SOME_DEFAULT_KEY_NAME" that gets stored in a dict - it would overwrite 
the default value.

So it has to bypass __setitem__, as otherwise it can't distinguish 
between "real" and the default value - the latter one is not allowed to 
have a key that is in any imaginable way used by the user.

Diez