I needed to import some plugin code written in Python from a directory whose path isn’t known until runtime. Further, I needed a class object that was a subclass of the plugin base class.

from somewhere import PluginBase

class SomePlugin(PluginBase):
    def f1(self): ...
    def f2(self): ...

You can use the imp module to actually load the module from impl_dir. Note that impl_dir needs to be tem­porar­i­ly prepended to sys.path. Then you can find the plugin subclass using dir and issubclass.

import os, imp

def import_class(implementation_filename, base_class):
    impl_dir, impl_filename = os.path.split(implementation_filename)
    module_name, _ = os.path.splitext(impl_filename)

    try:
        sys.path.insert(0, impl_dir)
        fp, filename, description = imp.find_module(module_name)
        module = imp.load_module(module_name, fp, filename, description)
        for name in dir(module):
            obj = getattr(module, name)
            if (type(obj) == type(base_class)
                and issubclass(obj, base_class)
                and obj != base_class):
                    return obj
        raise ValueError("No subclass of {0} in {1}".format(
                base_class.__name__, implementation_filename))
    finally:
        sys.path.pop(0)