[Zope-Coders] ZopeVersionControl

[Shane Hathaway]

Florent Guillaume wrote:
> I was reading the diff for that change
> (http://cvs.zope.org/Products/ZopeVersionControl/ZopeRepository.py.diff?r1=1.2&r2=1.3),
> and Shane wrote:
> 
>    try:
>        v = isinstance(object, ObjectManager)
>    except TypeError:
>        # Python 2.1 isinstance() dislikes ExtensionClass instances.
>        # This is an adequate workaround.
>        v = 0
> 
> As I depend on this in some of my code, I'm wondering, in what case does
> this fail? I haven't found one:

In Python 2.1, if the second argument to isinstance() is an 
ExtensionClass, it raises a TypeError instead of returning false.  (In 
all the cases you tested, isinstance() returned true.)


[shane@shane python]$ python2.1
Python 2.1.3 (#1, Apr  9 2002, 10:20:47)
[GCC 2.96 20000731 (Mandrake Linux 8.1 2.96-0.63.1mdk)] on linux2
Type "copyright", "credits" or "license" for more information.
 >>> from ExtensionClass import Base
 >>> class foo(Base):
...   pass
...
 >>> class c:
...   pass
...
 >>> isinstance(foo(), Base)
1
 >>> isinstance(c(), Base)
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
TypeError: isinstance() arg 2 must be a class or type


Shane