isinstance() not recognizing an instance correctly (python bug?)

[Fernando Perez]

Hi all,

I've found a rather unpleasant situation where isinstance(obj,cls)==0, even 
though obj _is_ indeed a cls() instance. I don't know if this should be 
considered a bug or just an unavoidable consequence of the circumstances 
where it happens.

As part of my IPython interactive interpreter shell, the system that takes 
care of displaying results (a simple 'print' in the default case) can look 
for certain classes. Now consider that interactively I do:

>>> from somemodule import someclass
>>> s=someclass()
>>> s
{ at this point, the system needs to print the object s }

In the printing code, suppose the display() function below is what gets 
called:

from somemodule import someclass

...

def display(arg):
  if isinstance(arg,someclass):
     print_fancy(arg)
  else:
     print arg


Well,  when 's' above is passed to display(), it fails to identify that s is 
indeed an instance of someclass(). The reason, I think, is that the 
interactive code is executed in a separate namespace than the display code, 
and therefore someclass exists with two different memory addresses. So maybe 
there's just no way out of the problem.

But I think it's a subtle enough 'surprising behavior' that it should at least 
be mentioned in the docs that 'isinstance(i,c) returns true if i is an 
instance of class c, except when it gets confused and gives the wrong answer. 
Use it and hope for the best.' ;)

Seriously though, while somewhat particular the circumstances where it fails 
aren't really that strange, and this failure mode should definitely be 
documented, if not considered a bug.

Cheers,

f.