isinstance check of variable and module

Question

I have a list of strings, which all are modules that are imported (I parsed the __init__.py file earlier for some other validation). For example mods = ['mod_one', 'mod_two', 'mod_three']. I now need to verify that all of these are an instance of a pre-defined superclass (skeleton.py) - like

for x in mods:
    if not isinstance(x.Operation,skeleton.OperationSkeleton): 
        print("error with: "+x)`

My problem is that x is in this case a string - obviously, but is the name of the imported module I want to check. The string doesn't have a Operation..

And odd enough.. for testing purpose did I try to

print(isinstance(mod_one.Operation,skeleton.OperationSkeleton))

and it prints False. But in mod_one.py the OperationSkeleton is imported from skeleton and class Clustering(ClusteringSkeleton): is created.

class Clustering is not the same thing as class.Operation. — Martijn Pieters
– Martijn Pieters, Commented Oct 21, 2014 at 12:50
whoops. thanks for editing. But in mod_one.py the OperationSkeleton is imported from skeleton and class Operation(OperationSkeleton): is created. — Daedalus Mythos
– Daedalus Mythos, Commented Oct 21, 2014 at 13:01
Right, in which case isinstance() would just work. But do make sure you didn't accidentally added a package subdirectory to your path and imported the module under two different names (package.module and module), for example. You can verify you have the same class by looking at the id(skeleton.OperationSkeleton) and also look at skeleton.__name__ and skeleton.OperationSkeleton.__module__ in both locations. — Martijn Pieters
– Martijn Pieters, Commented Oct 21, 2014 at 13:03

Martijn Pieters · Accepted Answer · 2014-10-21 13:29:20Z

1

If your modules are already imported, you can access them by their name in the sys.modules mapping:

import sys

for x in mods:
    mod = sys.modules[x]
    if not issubclass(mod.Operation, skeleton.OperationSkeleton):

You need to use issubclass() here as Operation is not an instance.

edited Oct 21, 2014 at 13:29

answered Oct 21, 2014 at 12:51

Martijn Pieters

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6 Comments

Daedalus Mythos Over a year ago

works perfectly fine! But I still get a False on print(isinstance(sys.modules["my_modules.mod_one"].Operation, skeleton.OperationSkeleton))

Martijn Pieters Over a year ago

@DaedalusMythos: see my comment on your question; clearly you are not using the same class object to base the Operation class on as what you are using for isinstance().

Martijn Pieters Over a year ago

@DaedalusMythos: Another way you could introspect this is sys.modules["my_modules.mod_one"].Operation.__bases__; that is a tuple of base classes for Operation. Again look at things like the output of id() to see what objects are actually being used here.

Daedalus Mythos Over a year ago

I'm a little overwhelmed but grateful for the help! I learned that both __name__s use the my_modules. in front. But even if i print(isinstance(sys.modules["my_modules.mod_one"].Operation, sys.modules["my_modules.skeleton"].OperationSkeleton) it's False. What am I missing?

Martijn Pieters Over a year ago

@DaedalusMythos: Sorry, I am being slow. You don't have an instance, you have a class, so use issubclass().

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