Monkey-patch Python class

import module
class ReplaceClass(object):
    ....
module.MyClass = ReplaceClass

Avoid the from ... import (horrid;-) way to get barenames when what you need most often are qualified names. Once you do things the right Pythonic way:

import module

class ReplaceClass(object): ...

module.MyClass = ReplaceClass

This way, you're monkeypatching the module object, which is what you need and will work when that module is used for others. With the from ... form, you just don't have the module object (one way to look at the glaring defect of most people's use of from ...) and so you're obviously worse off;-);

The one way in which I recommend using the from statement is to import a module from within a package:

from some.package.here import amodule

so you're still getting the module object and will use qualified names for all the names in that module.

I am but an egg . . . . Perhaps it is obvious to not-newbies, but I needed the from some.package.module import module idiom.

I had to modify one method of GenerallyHelpfulClass. This failed:

import some.package.module

class SpeciallyHelpfulClass(some.package.module.GenerallyHelpfulClass): 
    def general_method(self):...

some.package.module.GenerallyHelpfulClass = SpeciallyHelpfulClass

The code ran, but didn't use the behaviors overloaded onto SpeciallyHelpfulClass.

This worked:

from some.package import module

class SpeciallyHelpfulClass(module.GenerallyHelpfulClass): 
    def general_method(self):...

module.GenerallyHelpfulClass = SpeciallyHelpfulClass

I speculate that the from ... import idiom 'gets the module', as Alex wrote, as it will be picked up by other modules in the package. Speculating further, the longer dotted reference seems to bring the module into the namespace with the import by long dotted reference, but doesn't change the module used by other namespaces. Thus changes to the import module would only appear in the name space where they were made. It's as if there were two copies of the same module, each available under slightly different references.