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

  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.


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