Can I add attributes to class methods in Python?

Question

I have a class like this:

class A:
    def __init__(self):
        self.size=0
    def change_size(self,new):
        self.size=new

I want to add an attribute to the change_size method to say what it changes - i.e. so that

A(blah)
blah.change_size.modifies

returns

'size'

is this possible? I have tried:

class A:
    def __init__(self):
        self.size=0
    def change_size(self,new):
        self.change_size.modifies = 'size'
        self.size=new

nope

class A:
    def __init__(self):
        self.size=0
        self.change_size.modifies = 'size'
    def change_size(self,new):
        self.size=new

nope

class A:
    def __init__(self):
        self.size=0
    def change_size(self,new,modifies='size'):
        self.size=new

none of which seem to work.

Possible duplicate of How to add property to a class dynamically? — Shrey
– Shrey, Commented Jan 8, 2018 at 6:37
The problem you're running into is that self.change_size is a wrapper that passes self to the method, not the method itself. You'll need to do this through the class, not an instance. — kindall
– kindall, Commented Jan 8, 2018 at 6:50

user2357112 · Accepted Answer · 2018-01-08 06:36:08Z

3

That's simple enough. It goes basically the same way you'd add attributes to any other function:

class A:
    def __init__(self):
        self.size=0
    def change_size(self,new):
        self.size=new
    change_size.modifies = 'size'

print(A.change_size.modifies) # prints size

answered Jan 8, 2018 at 6:36

user2357112

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1 Comment

Ben Over a year ago

well damn... that was simple! You've solved my problem so fast here that I have to wait before I'm allowed to mark your answer as correct haha

Robert Nyström · Accepted Answer · 2020-07-24 06:56:44Z

3

A more universal solution with a help decorator.

from functools import wraps

def attributes(**attrs):
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            return f(*args, **kwargs)

        for attr_name, attr_value in attrs.items():
            setattr(wrapper, attr_name, attr_value)

        return wrapper

    return decorator

and rewrite you class as

class A:
    def __init__(self):
        self.size = 0

    @attributes(modifies='size')
    def change_size(self, new):
        self.size = new

edited Jul 24, 2020 at 6:56

Robert Nyström

1157 bronze badges

answered Jan 8, 2018 at 8:57

Dectinc

695 bronze badges

3 Comments

user2357112 Over a year ago

My answer is not setting a class attribute. Setting a class attribute would be something like change_size_modifies = 'size' at class scope instead of change_size.modifies = 'size'. Also, the wrapper in your decorator is unnecessary (you could just set the attributes on the original f and return f), and your syntax is invalid; you shouldn't quote modifies.

Dectinc Over a year ago

Thanks for reminding the syntax error, the previous answer is updated.

user2357112 Over a year ago

Your wrapper is passing the wrong arguments to f; it should be f(*args, **kwargs), not f(args, kwargs).

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