[Python-Dev] RE: PySequence_Contains (original) (raw)

Tim Peters tim.one@home.com
Sat, 5 May 2001 17:24:58 -0400

[Guido]

Actually, instancecontains checks for AttributeError only after calling instancegetattr(), whose only purpose is to return the requested attribute or raise AttributeError, so here it is safe: the contains function hasn't been called yet.

I'd say "safer", but not "safe": at that point we only know that some attribute didn't exist, somewhere, while attempting to look up "contains". Ignoring it could, e.g., be masking a bug in a getattr hook, like

def __getattr__(self, attr):
    return global_resolver.resolve(self, attr)

where global_resolver has lost its "resolve" attr. "except" clauses aren't more bulletproof in C than in Python <0.9 wink>.

With previous behavior of 'x in instance'. Before we had contains, 'x in y' always iterated over the items of y as a sequence, comparing them to x one at a time.

I don't believe I ever knew that! Thanks. I erronesouly assumed that the looping behavior was introduced when contains was added.

... No, that was probably just an oversight -- clearly it should have used rich comparisons. (I guess this is a disadvantage of the approach I'm recommending here: if the default behavior changes, the reimplementation of the default behavior in the class must be changed too.)

I factored out the new iterator-based contains logic into a new private API function, called when appropriate by both PySequence_Contains() and instance_contains(). So any future changes to what iterator-based contains means will only need to be made in one place.

too-easy-ly y'rs - tim