Python decorators with class methods

Question

I want a decorator that I can use as both @decorator and decorator() with class methods, like this :

def decorator_with_args(name):
    print(f'Hello {name} !')
    def decorator(func):
        def wrapper(self, *args, **kwargs):
            print(f'Hello {self.title} {name} again !!')
            print(f"Calling {func.__name__} with instance {self}...")
            result = func(self, *args, **kwargs)
            print(f"{func.__name__} finished. Result: {result}")
            return result
        return wrapper
    return decorator

class Calculator:
    def __init__(self):
        self.title = 'Mr'

    @decorator_with_args('World')
    def add(self, a, b):
        return a + b

    def add2(self, a, b):
        return a + b

    def do_add(self, a, b):
        return decorator_with_args(f'World{a}')(self.add2)(self, a, b)

# Usage 1
calc = Calculator()
result = calc.add(3, 5)

# Usage 2
calc = Calculator()
result = calc.do_add(3, 5)

The reason why I want to use the decorator function as the above two representations is because:

I want to use @decorator_with_args when the argument is known such as @decorator_with_args('World')
I want to use decorator_with_args() when the argument is not known and I want the argument to be dynamic such as decorator_with_args(f'World{a}')

While @decorator_with_args works as expected [Usage 1], I get an error with decorator_with_args() [Usage 2]. I tried few things, but none of them worked for the latter.

If I try to pass (self, a, b), for example decorator_with_args(f'World{a}')(self.add2)(self, a, b), I get TypeError: add2() takes 3 positional arguments but 4 were given.

On the other hand, if I try to pass (a, b) without self, for example decorator_with_args(f'World{a}')(self.add2)(a, b), I get AttributeError: 'int' object has no attribute 'title'.

I appreciate there may exist other similar questions, which I tried to search but could not get them to work for my use case.

matszwecja · Accepted Answer · 2024-06-25 11:47:37Z

4

In order to replicate the standard decorating of instance method, you need to decorate the underlying function that is bound to that method, not the method itself - that is because self.add2 implicitly adds self as first argument to that function, so it gets doubled.

    def do_add(self, a, b):
        return decorator_with_args(f'World{a}')(Calculator.add2)(self, a, b)

or

    def do_add(self, a, b):
        return decorator_with_args(f'World{a}')(self.add2.__func__)(self, a, b)

answered Jun 25, 2024 at 11:47

matszwecja

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Marco Bresson · Accepted Answer · 2024-06-25 12:04:52Z

If you add print(f"Calling {func} with instance {self}...") in your wrapper, you will see the following:

# Usage 1
calc = Calculator()
result = calc.add(3, 5)
>>> Calling <function Calculator.add at 0x15815b5b0> with instance <__main__.Calculator object at 0x158174430>...
>>> add finished. Result: 8

# Usage 2
calc = Calculator()
result = calc.do_add(3, 5)
>>> Calling <bound method Calculator.add2 of <__main__.Calculator object at 0x158177100>> with instance <__main__.Calculator object at 0x158177100>...
>>> TypeError...

This is because self.add2 is bound to the instance and so your wrapper will have 4 arguments:

the "self" from the bound method
the "self" from the function call in do_add
a
b

If you want to access the function from the bound method, you can do the following:

def do_add(self, a, b):
    return decorator_with_args(f'World{a}')(Calculator.add2)(self, a, b)

>>> Hello World3 !
>>> Hello Mr World3 again !!
>>> Calling <function Calculator.add2 at 0x15815b370> with instance <__main__.Calculator object at 0x1581be860>...
>>> <__main__.Calculator object at 0x1581be860> 3 5
>>> add2 finished. Result: 8

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