self lost when using partial inside a decorator

Question

I'm trying to code a method from a class that uses a decorator from another class. The problem is that I need information stored in the Class that contains the decorator (ClassWithDecorator.decorator_param). To achieve that I'm using partial, injecting self as the first argument, but when I do that the self, from the class that uses the decorator " gets lost" somehow and I end up getting an error. Note that this does not happen if I remove partial() from my_decorator() and "self" will be correctly stored inside *args.

See the code sample:

from functools import partial


class ClassWithDecorator:
    def __init__(self):
        self.decorator_param = "PARAM"

    def my_decorator(self, decorated_func):
        def my_callable(ClassWithDecorator_instance, *args, **kwargs):
            # Do something with decorator_param
            print(ClassWithDecorator_instance.decorator_param)
            return decorated_func(*args, **kwargs)

        return partial(my_callable, self)


decorator_instance = ClassWithDecorator()


class WillCallDecorator:
    def __init__(self):
        self.other_param = "WillCallDecorator variable"

    @decorator_instance.my_decorator
    def decorated_method(self):
        pass


WillCallDecorator().decorated_method()

I get

PARAM
Traceback (most recent call last):
  File "****/decorator.py", line 32, in <module>
    WillCallDecorator().decorated_method()
  File "****/decorator.py", line 12, in my_callable
    return decorated_func(*args, **kwargs)
TypeError: decorated_method() missing 1 required positional argument: 'self'

How can I pass the self corresponding to WillCallDecorator() into decorated_method() but at the same time pass information from its own class to my_callable() ?

Does this answer your question? [Decorators with parameters?](https://stackoverflow.com/questions/5929107/decorators-with-parameters) — Energya, Dec 20 '19 at 12:43
The decorator works always on the unbound method which needs to receive the self explicitly as parameter yet. — Michael Butscher, Dec 20 '19 at 13:05
Why are you using `partial` at all? The decorator can access `self` directly as a scoped variable. — Duncan, Dec 20 '19 at 14:37

score 5 · Answer 1 · answered Dec 20 '19 at 13:05

5

It seems that you may want to use partialmethod instead of partial:

From the docs:

class functools.partialmethod(func, /, *args, **keywords)

When func is a non-descriptor callable, an appropriate bound method is created dynamically. This behaves like a normal Python function when used as a method: the self argument will be inserted as the first positional argument, even before the args and keywords supplied to the partialmethod constructor.

answered Dec 20 '19 at 13:05

Energya

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That is exactly what I wanted, thanks. next time I will read the full official docs – Gonzalo Hernandez Dec 20 '19 at 13:23

Duncan · Accepted Answer · 2019-12-20T14:51:02.417

So much simpler just to use the self variable you already have. There is absolutely no reason to be using partial or partialmethod here at all:

from functools import partial


class ClassWithDecorator:
    def __init__(self):
        self.decorator_param = "PARAM"

    def my_decorator(self, decorated_func):
        def my_callable(*args, **kwargs):
            # Do something with decorator_param
            print(self.decorator_param)
            return decorated_func(*args, **kwargs)

        return my_callable


decorator_instance = ClassWithDecorator()


class WillCallDecorator:
    def __init__(self):
        self.other_param = "WillCallDecorator variable"

    @decorator_instance.my_decorator
    def decorated_method(self):
        pass


WillCallDecorator().decorated_method()

Also, to answer your question about why your code didn't work, when you access something.decorated_method() the code checks whether decorated_method is a function and if so turns it internally into a call WillCallDecorator.decorated_method(something). But the value returned from partial is a functools.partial object, not a function. So the class lookup binding won't happen here.

In more detail, something.method(arg) is equivalent to SomethingClass.method.__get__(something, arg) when something doesn't have an attribute method and its type SomethingClass does have the attribute and the attribute has a method __get__ but the full set of steps for attribute lookup is quite complicated.

self lost when using partial inside a decorator

2 Answers2