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.