Python: why can isinstance return False, when it should return True?

Question

I'm currently in pdb trace to figure this out

ipdb> isinstance(var, Type)
False
ipdb> type(var)
<class 'module.Type'>
ipdb> Type
<class 'module.Type'>

Why can this happen?

P. S. isinstance(var, type(var)) returns True as expected

Answer 1

1. I can only guess, but if you do in module

   class Type(object): pass
   var = Type()
   class Type(object): pass
   

   then both types look like <class 'module.Type'>, but are nevertheless different.

   You could check that with

   print(id(Type), id(var.__class__))
   

   or with

   print(Type is var.__class__)
   

   Note that these comparisons work for both old-, and new-style classes. For new-style classes, they are equivalent to print(Type is type(var)). But this is not the case for old-style classes.

2. Another quite common trap is that you call this module with

   python -m module
   

   or

   python module.py
   

   making it known as __main__ module. If it is imported somewhere else under its true name, it is known under that name as well with a different name space.

3. Another guess could be that you are working with ABCs or otherwise let the class have a __instancecheck__() method.

Answer 2

Users of the "autoreload" setting in iPython will probably run into this. When a class is reloaded, it will become a new class with the exact same name. Instances of the old class will not have their type updated:

# After reloading the Role class

[ins] In [19]: y = Role()

[ins] In [20]: isinstance(x, Role)
Out[20]: False

[nav] In [21]: isinstance(y, Role)
Out[21]: True

[ins] In [22]: type(x)
Out[22]: myproject.auth.Role

[ins] In [23]: type(y)
Out[23]: myproject.auth.Role