Using the same decorator (with arguments) with functions and methods.

Question

I have been trying to create a decorator that can be used with both functions and methods in python. This on it's own is not that hard, but when creating a decorator that takes arguments, it seems to be.

class methods(object):
    def __init__(self, *_methods):
        self.methods = _methods

    def __call__(self, func): 
        def inner(request, *args, **kwargs):
            print request
            return func(request, *args, **kwargs)
        return inner

    def __get__(self, obj, type=None):
        if obj is None:
            return self
        new_func = self.func.__get__(obj, type)
        return self.__class__(new_func)

The above code wraps the function/method correctly, but in the case of a method, the request argument is the instance it is operating on, not the first non-self argument.

Is there a way to tell if the decorator is being applied to a function instead of a method, and deal accordingly?

Answer 1

The decorator is always applied to a function object -- have the decorator print the type of its argument and you'll be able to confirm that; and it should generally return a function object, too (which is already a decorator with the proper __get__!-) although there are exceptions to the latter.

I.e, in the code:

class X(object):

  @deco
  def f(self): pass

deco(f) is called within the class body, and, while you're still there, f is a function, not an instance of a method type. (The method is manufactured and returned in f's __get__ when later f is accessed as an attribute of X or an instance thereof).

Maybe you can better explain one toy use you'd want for your decorator, so we can be of more help...?

Edit: this goes for decorators with arguments, too, i.e.

class X(object):

  @deco(23)
  def f(self): pass

then it's deco(23)(f) that's called in the class body, f is still a function object when passed as the argument to whatever callable deco(23) returns, and that callable should still return a function object (generally -- with exceptions;-).

Answer 2

Since you're already defining a __get__ to use your decorator on the Bound Method, you could pass a flag telling it if it's being used on a method or function.

class methods(object):
    def __init__(self, *_methods, called_on_method=False):
        self.methods = _methods
        self.called_on_method

    def __call__(self, func):
        if self.called_on_method:
            def inner(self, request, *args, **kwargs):
                print request
                return func(request, *args, **kwargs)
        else:
            def inner(request, *args, **kwargs):
                print request
                return func(request, *args, **kwargs)
        return inner

    def __get__(self, obj, type=None):
        if obj is None:
            return self
        new_func = self.func.__get__(obj, type)
        return self.__class__(new_func, called_on_method=True)

Answer 3

A partial (specific) solution I have come up with relies on exception handling. I am attempting to create a decorator to only allow certain HttpRequest methods, but make it work with both functions that are views, and methods that are views.

So, this class will do what I want:

class methods(object):
    def __init__(self, *_methods):
        self.methods = _methods

    def __call__(self, func): 
        @wraps(func)
        def inner(*args, **kwargs):
            try:
                if args[0].method in self.methods:
                    return func(*args, **kwargs)
            except AttributeError:
                if args[1].method in self.methods:
                    return func(*args, **kwargs)
            return HttpResponseMethodNotAllowed(self.methods)
        return inner

Here are the two use cases: decorating a function:

@methods("GET")
def view_func(request, *args, **kwargs):
    pass

and decorating methods of a class:

class ViewContainer(object):
    # ...

    @methods("GET", "PUT")
    def object(self, request, pk, *args, **kwargs):
        # stuff that needs a reference to self...
        pass

Is there a better solution than to use exception handling?

Answer 4

To expand on the __get__ approach. This can be generalized into a decorator decorator.

class _MethodDecoratorAdaptor(object):
    def __init__(self, decorator, func):
        self.decorator = decorator
        self.func = func
    def __call__(self, *args, **kwargs):
        return self.decorator(self.func)(*args, **kwargs)
    def __get__(self, instance, owner):
        return self.decorator(self.func.__get__(instance, owner))

def auto_adapt_to_methods(decorator):
    """Allows you to use the same decorator on methods and functions,
    hiding the self argument from the decorator."""
    def adapt(func):
        return _MethodDecoratorAdaptor(decorator, func)
    return adapt

In this way you can just make your decorator automatically adapt to the conditions it is used in.

def allowed(*allowed_methods):
    @auto_adapt_to_methods
    def wrapper(func):
        def wrapped(request):
            if request not in allowed_methods:
                raise ValueError("Invalid method %s" % request)
            return func(request)
        return wrapped
    return wrapper

Notice that the wrapper function is called on all function calls, so don't do anything expensive there.

Usage of the decorator:

class Foo(object):
    @allowed('GET', 'POST')
    def do(self, request):
        print "Request %s on %s" % (request, self)

@allowed('GET')
def do(request):
    print "Plain request %s" % request

Foo().do('GET')  # Works
Foo().do('POST') # Raises