getting special from type, not instance (was Re: [Python-Dev] copy confusion) (original) (raw)

Guido van Rossum gvanrossum at gmail.com
Wed Jan 12 18:59:13 CET 2005

[Alex]

Armin's fix was to change:

conform = getattr(type(obj), 'conform', None) into: for basecls in type(obj).mro: if 'conform' in basecls.dict: conform = basecls.dict['conform'] break else: # not found I have only cursorily examined the rest of the standard library, but it seems to me there may be a few other places where getattr is being used on a type for this purpose, such as pprint.py which has a couple of occurrences of r = getattr(typ, "repr", None) [And then proceeds to propose a new API to improve the situation]

I wonder if the following solution wouldn't be more useful (since less code will have to be changed).

The descriptor for getattr and other special attributes could claim to be a "data descriptor" which means that it gets first pick even if there's also a matching entry in the instance dict. Quick illustrative example:

class C(object): foo = property(lambda self: 42) # a property is always a "data descriptor"

a = C() a.foo 42 a.dict["foo"] = "hello" a.foo 42

Normal methods are not data descriptors, so they can be overridden by something in dict; but it makes some sense that for methods implementing special operations like getitem or copy, where the instance dict is already skipped when the operation is invoked using its special syntax, it should also be skipped by explicit attribute access (whether getattr(x, "getitem") or x.getitem -- these are entirely equivalent).

We would need to introduce a new decorator so that classes overriding these methods can also make those methods "data descriptors", and so that users can define their own methods with this special behavior (this would be needed for copy, probably).

I don't think this will cause any backwards compatibility problems -- since putting a getitem in an instance dict doesn't override the x[y] syntax, it's unlikely that anybody would be using this. "Ordinary" methods will still be overridable.

PS. The term "data descriptor" now feels odd, perhaps we can say "hard descriptors" instead. Hard descriptors have a set method in addition to a get method (though the set method may always raise an exception, to implement a read-only attribute).

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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