Creating an object and passing an user-defined function as method

Question

Let us suppose the following situation: I have a class with some initial values. Furthermore, I want to provide the possibility to pass an user-defined method, when initializing a new object. The user knows about the attributes of the class in advance and may want to consider them in the function, for instance:

class some_class():
    def __init__(self, some_method):
        # some initial values
        self.a = 8
        self.b = 12
        # initializing a new object with a user-specific method
        self.some_method = some_method 

    def some_method(self):
        pass # this method shall be specified by the user

# user-specific function
def some_function(): 
    return self.a + self.b

some_object = some_class(some_method = some_function)
print(some_object.some_method())

Of course, the given example does not work, but I hope it shows what I want to do. I am searching for a way to define a function outside the class, which refers to the attribute of an object after it was passed during initialization.

What I try to avoid is to solve the problem with fixed name conventions, for instance:

class some_class():
    def __init__(self, some_method):
        self.a = 8
        self.b = 12
        self.some_method = some_method 

    def some_method(self):
        pass

def some_function(): 
    return some_object.a + some_object.b # -> fixed names to avoid the problem

some_object = some_class(some_method = some_function)
print(some_object.some_method())

I think what I need is a kind of placeholder or alternative to self. Does anybody has an idea?

If you want the user to supply additional code when using your class, they should be subclassing it rather than merely instantiating it. — jasonharper, Apr 29 '19 at 15:01
What you want to do is binding a function to an object. See this question: https://stackoverflow.com/questions/1015307/python-bind-an-unbound-method — blues, Apr 29 '19 at 15:03
Indeed but it is doable. I mean, this is python, all kinds of loopyland things are possible. — Josef Korbel, Apr 29 '19 at 15:04
Thank you all for the many advices. However, blues is right. After following his link, I recognized that it was exactly, what I have searched. I just didn't know how it is called, what I've searched for — Slango, Apr 30 '19 at 18:23

score 6 · Answer 1 · answered Apr 29 '19 at 15:03

6

This works, although not sure it is the most elegant way to achieve what you want:

class some_class():
    def __init__(self, some_method):
        self.a = 8
        self.b = 12
        self.some_method_func = some_method

    def some_method(self):
        return self.some_method_func(self)

def some_function(self):
    return self.a + self.b

some_object = some_class(some_method = some_function)
print(some_object.some_method())

answered Apr 29 '19 at 15:03

vladmihaisima

2,119
16
20

1

The proper way to do that is binding the function. https://stackoverflow.com/questions/1015307/python-bind-an-unbound-method – blues Apr 29 '19 at 15:04

score 0 · Answer 2 · answered Apr 29 '19 at 15:02

If I'm reading this right, then the easy option is just to do the following:

class some_class():
    def __init__(self):
        self.a = 8
        self.b = 12

    def some_method(self):
        pass

def my_method(self):
    return self.a + self.b

# Redefine the class method
some_class.some_method = my_method

# Or, if you only want to do it for a specific instance:

instance = some_class()
instance.some_method = my_method

The usual way to do this 'properly' though is with sub-classing.

score 0 · Answer 3 · answered Apr 29 '19 at 15:06

You need to get the class and the caller of the class to agree to a contract. The contract is that the class will pass in the instance of the class to the function, and the function must accept that as an argument.

class some_class():
    def some_method(self):
        return self.some_method_func(self)

def some_function(obj):
   return obj.a + obj.b

Creating an object and passing an user-defined function as method

3 Answers3