[Python-bugs-list] [ python-Bugs-532767 ] isinstance() should respect class (original) (raw)

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Tue, 11 Feb 2003 19:41:06 -0800

Bugs item #532767, was opened at 2002-03-20 17:22 You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=532767&group_id=5470

Category: Type/class unification Group: Python 2.3

Status: Closed Resolution: Accepted Priority: 6 Submitted By: Steve Alexander (stevea_zope) Assigned to: Guido van Rossum (gvanrossum) Summary: isinstance() should respect class

Initial Comment: isinstance(obj, class_or_type_or_tuple) should compare using obj.class when obj is an instance of a type or a new-style class.

This is important for using weak references and other kinds of proxy wrappers, where you want to pass a proxy to some code, which might query its type using isinstance.

issubclass may need a similar treatment.

Comment By: Guido van Rossum (gvanrossum) Date: 2003-02-11 22:41

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OK, checked in. When X is a new-style class, isinstance(x, X) is now defined as "issubclass(type(x), X) or issubclass(x.class, X)". That seems a very reasonable definition to me.

Comment By: Guido van Rossum (gvanrossum) Date: 2002-03-21 09:52

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I'm not sure I like SteveA's suggestion of making isinstance() respond both to the wrapper type and to the wrapped type. Although there's a precedent:

class C: pass # classic class ... c = C() isinstance(c, C) 1 import types isinstance(c, types.InstanceType) 1

I'm also not sure I agree with Martin's assertion that type(o) is X and isinstance(o, X) should always be equivalent. Especially with new-style user-defined classes, we could also require that o.class == X and isinstance(o, X) should be equivalent (modulo subclassing), and that would require isinstance() to prefer class.

I guess we'll have to look at actual use cases of isinstance() and wrappers (both weakrefs and Zope3's transparent wrappers). My expectation is that, since wrappers try hard to pretend to be the wrapped object, extending this to the isinstance() test is more useful than the stricter interpretation.

Comment By: Steve Alexander (stevea_zope) Date: 2002-03-21 08:57

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oops... please ignore my unsupported use of "is" to compare small ints.

I meant:

from Zope.ContextWrapper import Wrapper

wl = Wrapper([]) assert isinstance(wl, list) == 1 assert isinstance(wl, Wrapper) == 1

Comment By: Steve Alexander (stevea_zope) Date: 2002-03-21 08:55

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A new isinstance can maintain and extend the semantic Martin describes.

Let's say object wl is a wrapped list:

from Zope.ContextWrapper import Wrapper

wl = Wrapper([]) assert isinstance(wl, list) is 1 assert isinstance(wl, Wrapper) is 1

So, your semantics are maintained. With the proposed change, the property you describe need not be given up.

Comment By: Martin v. L�wis (loewis) Date: 2002-03-21 08:47

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There has been a long-standing guarantee that 'type(o) is X' implies 'isinstance(o, X)', or, more compact, 'isinstance(o,type(o))' for all objects o. In fact, people have been advised to change the explicit test for type() to isinstance calls. With the proposed change, this property will be given up.

Comment By: Guido van Rossum (gvanrossum) Date: 2002-03-21 08:16

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I give this a +1. To refute Martin's -1: The use case that prompts this is passing a wrapper to wrapper-ignorant code. This may be 3rd party code that you can't afford to make wrapper-aware. If I pass you a wrapper to an X where you expect an X, your isinstance(x, X) call should succeed. Especially since x.class already returns X. Also, isinstance(x, X) succeeds if X is a classic class and x is a wrapped X instance.

If you want to know if something is a wrapper, you have to use type().

Comment By: Martin v. L�wis (loewis) Date: 2002-03-21 03:59

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-1. That means that you can't use isinstance anymore to determine whether something is a weak reference, or other kind of proxy wrapper.

If you need a function that unwraps wrappers, write one yourself.

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