En Tue, 22 Jun 2010 23:45:07 -0300, John Nagle <na...@animats.com>
escribió:
I want to test whether an object is an instance of any user-defined
class. "isinstance" is less helpful than one would expect.
>>> import types
>>> class foo() : # define dummy class
... pass
...
>>> x = foo()
>>>
>>> type(x)
<type 'instance'>
>>>
>>> isinstance(x, types.ClassType)
False
>>> isinstance(x, types.InstanceType)
True
>>> foo
<class __main__.foo at 0x004A2BD0>
>>> x
<__main__.foo instance at 0x020080A8>
So far, so good. x is an InstanceType. But let's try a
class with a constructor:
>>> class bar(object) :
... def __init__(self, val) :
... self.val = val
...
>>> b = bar(100)
>>> b
<__main__.bar object at 0x01FF50D0>
>>> isinstance(b, types.InstanceType)
False
>>> isinstance(b, types.ClassType)
False
>>>>>> bar
<class '__main__.bar'>
Without a constructor, we get an "instance". With a constructor,
we get an "object", one which is not an InstanceType.
That's not the relevant difference. In the first case, you don't inherit
from object; in the second one, you do.
foo is a "classic" class (or "old-style" class); x is an instance of foo,
its *type* is InstanceType, its *class* is foo. All instances of any other
classic class have the same type (InstanceType).
bar is a "new-style" class, b is an instance of bar, its type is bar, its
class is bar. class and type are equivalent for new style classes; things
are a lot more regular and predictable. In Python 3.x classic classes are
gone.
--
Gabriel Genellina
--
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