method_decorator doesn't supports decorators with arguments

[Marinho Brandão]

I tried use django.utils.decorators.method_decorator to support decorator permission_required and figured out it's not supporting decorators passing arguments like it does:

@permission_required(something_here)

Instead its code waits for you just pass a function as an only one argument, like this:

@login_required

I made my own version of it, that's below:

from functools import wraps, update_wrapper

def method_decorator(decorator):
    """Converts a function decorator into a method decorator.
    
    This works properly for both: decorators with arguments and without them. The Django's version
    of this function just supports decorators with no arguments."""

    # For simple decorators, like @login_required, without arguments
    def _dec(func):
        def _wrapper(self, *args, **kwargs):
            def bound_func(*args2, **kwargs2):
                return func(self, *args2, **kwargs2)
            return decorator(bound_func)(*args, **kwargs)
        return wraps(func)(_wrapper)

    # Called everytime
    def _args(*argsx, **kwargsx):
        # Detect a simple decorator and call _dec for it
        if len(argsx) == 1 and callable(argsx[0]) and not kwargsx:
            return _dec(argsx[0])

        # Used for decorators with arguments, like @permission_required('something')
        def _dec2(func):
            def _wrapper(self, *args, **kwargs):
                def bound_func(*args2, **kwargs2):
                    return func(self, *args2, **kwargs2)
                return decorator(*argsx, **kwargsx)(bound_func)(*args, **kwargs)
            return wraps(func)(_wrapper)
        return _dec2

    update_wrapper(_args, decorator)
    # Change the name to aid debugging.
    _args.__name__ = 'method_decorator(%s)' % decorator.__name__
    return _args

If necessary I can make a patch and attach to this ticket.

[Natalia Bidart]

I'm not sure about the goal of this request, so I'll leave the design decision to the core devs.

[Łukasz Rekucki]

IMHO, the goal here is to be able to write:

m_perm_required = method_decorator(permission_required) # do this once

class A(View):
   @m_perm_required('spam.mores_spam') # use it later many times without repeating method_decorator 
   def get(self, request, *args, **kwrgs):
     #...

The current implementation handles only decorators without arguments, so you can only write:

   @method_decorator(permission_required('spam.more_eggs')
   def method(self, ...): 
       #...

This is similar to diffrence between "decorator_from_middleware" and "decorator_from_middleware_with_args" and I think the right solution here is to provide a "method_decorator_with_args" function.

PS. The OP is wrong that his implementation handles all cases - there is no universal pattern in Python for transforming decorators with and without arguments if you allow the first argument to be a callable (a class for example). It's probably a matter of taste, so you can just disregard this part of comment :)

[Luke Plant]

Technically, method_decorator works as advertised. permission_required is not actually a decorator. permission_required('spam.more_eggs') is a decorator, and permission_required is a decorator generator (it returns a decorator when you call it with some arguments).

But I agree that the goal set out lrekucki is a good one, and that a method_decorator_with_args (or some better name) would be desirable, and that the OP's solution is not a good idea.

[Andy Durdin]

Patch with fix and tests against trunk revision 16259

[Andy Durdin]

Implemented lukeplant's suggestion of method_decorator_with_args.

[Jannis Leidel]

I'm not terribly happy with the ..with_args suffix (same for decorator_from_middleware_with_args). It feels weird to having to remember two different function names that do basically the same. Especially for decorators that take arguments optionally I'd appreciate if the method_decorator function would simply allow me to pass the arguments for the decorator optionally, too. E.g.::

    @method_decorator(permission_required, 'spam.more_eggs', another_arg=True)
    def method(self, whatever):
        # bla

[Jannis Leidel]

After talking to adurdin on IRC more about it, I realized that this ticket shows why the "generate a method decorator" pattern is aweful. Marking as DDN.

[Luke Plant]

@jezdez: My experience with decorators tells me that confusing decorators and decorator factories is always a bad idea, despite the temptation of convenience. If you look at the ​cache_page decorator, you'll find probably some of the nastiest code in Django, and it was caused by this confusion. You always end up with isinstance checks and horrible little edge cases, and it makes it hard to reason about the code, since it can accept/return fundamentally different types of things.

However, in this case, this code:

    @method_decorator(permission_required, 'spam.more_eggs', another_arg=True)
    def method(self, whatever):

is no improvement over this code:

    @method_decorator(permission_required('spam.more_eggs', another_arg=True))
    def method(self, whatever):

in terms of the original motivation of this ticket, and it violates TOOWTDI.

[Jacob]

Milestone 1.3 deleted

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

I grudgingly acknowledge the opportunity of adding method_decorator_with_args, for consistency with decorator_from_middleware_with_args.

[Susan Tan]

Applied the attached patch and all the tests pass.