How to remove the self argument from a decorator's arguments if there is one

Question

Suppose I have a simple decorator, and a classes method and a function which I both decorate with that decorator:

import functools

def decorator(func):
    @functools.wraps(func)
    def call(*args):
        print(args)
        func(*args)
    return call

class cls:
    @decorator
    def meth(self, a):
        pass

@decorator
def func(c):
    pass

cls().meth("a")
func("c")

I get following output:

(<__main__.cls object at 0x7f4665c50130>, 'a')
('c',)

But I want to remove the self argument when the decorator is used on a method, so that the output becomes:

('a',)
('c',)

But, if I simply add args.pop(0), I will remove the first argument even if it is not self. How can I only remove the first argument if it is self ?

Note: I read some solutions with a long code using inspect - but there must be a shorter, easier way in this great and simple-to-use programming language ?

EDIT: Using @staticmethod is not an option for me, because I need the self parameter in the method itself. I only don't want it to get printed.

@May.D no, this is not an option for me; plus I don't see the benefit of it - I edited my question. — TheEagle, Jul 06 '21 at 23:48

srj · Accepted Answer · 2021-07-07T22:42:18.430

5

The simplest way to achieve this is to explicitly tell your decorator that it's decorating a class method. Your decorator will take a keyword argument denoting that.

def decorator(is_classmethod=False):
    def wrapper(func):
        @functools.wraps(func)
        def call(*args):
            if is_classmethod:
                print(args[1:])  # omit args[0]
            else:
                print(args)
            return func(*args)      # pass all the args to the function
        return call
    return wrapper

class cls:
    @decorator(is_classmethod=True)
    def meth(self, a):
        pass

@decorator
def func(c):
    pass

cls().meth("a")
func("c")

Since the user already knows what they're decorating, you can avoid complicated introspection code in the decorator, and give the user flexibility of processing the first argument or not.

please take a look at my answer to this SO question: Decorators with parameters? for a cleaner (IMHO) way to write decorators with parameters. There are some really good answers in that post.

edited Jul 07 '21 at 22:42

answered Jul 06 '21 at 23:50

srj

9,591
2
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Hmm … If I don't get another solution that auto-detects that, I'll accept this answer tomorrow ! +1 – TheEagle Jul 06 '21 at 23:50
`func("c")` print nothing – Andy Pavlov Jul 07 '21 at 00:29
You forgot to use `is_classmethod` to alter the behavior of `call`. – chepner Jul 07 '21 at 00:40
You could also replace `is_classmethod` with an integer parameter that gets used as the starting value of the slice directly. – chepner Jul 07 '21 at 00:42
updated the code example with the branch logic. @chepner, thanks for the review. – srj Jul 07 '21 at 22:42

Andy Pavlov · Answer 2 · 2021-07-07T00:24:27.183

1

You can filter args for output by existence of some python magic. For example:

import functools

def decorator(func):
    @functools.wraps(func)
    def call(*args):
        print(list(filter(lambda x: not hasattr(x, '__dict__'), args)))
        func(*args)
    return call

class cls:
    @decorator
    def meth(self, a):
        pass

@decorator
def func(c):
    pass

cls().meth("a")  # ['a']
func("c")  # ['c']

edited Jul 07 '21 at 00:24

answered Jul 07 '21 at 00:18

Andy Pavlov

316
2
8

This will not work if you pass any mutable class as the method arguments. `def meth(self, other_class_instance)` will get caught in the filter. – srj Jul 07 '21 at 22:46
Write please a simple example of such thing. I think that we can find solution) – Andy Pavlov Jul 07 '21 at 22:58
As I understand author need to see inputed variables and no other classes information. – Andy Pavlov Jul 07 '21 at 23:05
OP wants to avoid printing the `self` reference (which is the first argument for the function call if the function is a class or an instance method), and print all other arguments. if any of the arguments turns out to be an object implementing the `__dict__` attribute, it will get filtered by your decorator – srj Jul 08 '21 at 02:29
If I put `print(hasattr(self, '__dict__'))` in `cls.meth`, I get `True`. How does `filter(lambda x: not hasattr(x, '__dict__'), args)` work, then ? Please explain. – TheEagle Jul 08 '21 at 16:37

How to remove the self argument from a decorator's arguments if there is one

2 Answers2