[Python-bugs-list] [ python-Bugs-532767 ] isinstance() should respect __class__

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Bugs item #532767, was opened at 2002-03-20 17:22
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Category: Type/class unification
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
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.

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>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.

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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



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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.

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Comment By: Martin v. Löwis (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.

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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().

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Comment By: Martin v. Löwis (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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