Error Using Class Method Defined Outside of Class

Question

I have a class that I want to take an arbitrary method at initialization to use for string parsing depending on the context.

Using a method defined outside of a class as discussed here: Python define method outside of class definition?

def my_string_method(self):
    return self.var.strip()

class My_Class():
    def __init__(self, string_method):
        self.var = ' foo '
        self.string_method = string_method

    def use_string_method(self):
        return self.string_method()

instance = My_Class(string_method=my_string_method)
print instance.use_string_method()

I get the error "TypeError: use_string_method() takes exactly 1 argument (0 given)".

Shouldn't the self argument be passed implicitly to use_string_method? Is there a way to define the function such that this occurs, or do I need to explicitly pass the self argument to methods defined outside of the class as such:

class My_Class():
    def __init__(self, string_method):
        self.var = ' foo '
        self.string_method = string_method

    def use_string_method(self):
        return self.string_method(self)

Ouch, that's tricky. I had something similar, can't find a workaround :( — linusg, May 07 '16 at 16:09

score 1 · Accepted Answer · answered May 07 '16 at 16:13

1

You will have to wrap the passed in function in "MethodType".

From within your init:

self.string_method = types.MethodType(string_method, self)

This binds the method to the class and allows it to receive the implicit self parameter. Make sure you import types at the top of your script.

answered May 07 '16 at 16:13

Brian Schlenker

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The other option is to wrap it in `functools.partial()` or a lambda that passes self for you. – Kevin May 07 '16 at 16:33

Error Using Class Method Defined Outside of Class

1 Answers1