Injecting methods into instance / class

duncan smith duncan at invalid.invalid
Sun Dec 2 12:56:25 EST 2018

      I have a lot of functions that take an instance of a particular
class as the first argument. I want to create corresponding methods in
the class. I have tried the following, which (when called from __init__)
creates the relevant methods in an instance (Python 3.6).

def init_methods(self):
    for func_name, method_name in [('add', '__add__'),
                                   ('subtract', '__sub__')]:
        setattr(self, method_name,
types.MethodType(globals()[func_name], self))

The problem is that e.g.


works as expected, but

x - y

does not (because special methods are looked up in the class rather than
the instance).

I have tried to find examples of injecting methods into classes without
success. I have tried a few things that intuition suggested might work,
but didn't - like removing the first line above, dedenting and replacing
self with the class. This is only to save typing and make the code
cleaner, but I would like to get it right. Any pointers appreciated. TIA.


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