Hi,

This is more a question/proposal than a real issue. I apologize in
advance if this has already been debated or if it is not relevant.

I noticed `setdefault` method of mutable mapping objects is returning
the default value as passed in the call, when applicable, without
passing through `__getitem__`.

I think it would be more "symmetric" if it would return the default
value set by `__setitem__` via `__getitem__`. This would handle the
case of a custom MutableMapping defining a custom behaviour of
`__setitem__` and `__getitem__`.

Of course, in my case finally I overloaded `setdefault` to do this,
but it might be worth in general, at least for the MutableMapping
class.

Thanks,
Matias.

The current code makes sense to me and also mirrors the actual implementation of setdefault() for regular dictionaries. 

While I can see what you're trying do, it is a feature that __getitem__() is not called.  And for regular dicts, that is the only way to make it an atomic operation.

In any case, the proposal would be a behavior change that breaks code — users are likely and justifiably relying on the actual default value being returned rather than a possibly new value computed by the __getitem__() method.

There is also an efficiency concern.  Changing the behavior implies that missing keys would trigger one initial search, then a store for the default, and then another subsequent lookup.  Speaking only for myself, I know that I would never want that by default.  If required, I would prefer to override setdefault() and explicitly spell-out the new behavior.

I'm with Raymond. Closing.