Attaching a decorator to all functions within a cl

Question

I don't really need to do this, but was just wondering, is there a way to bind a decorator to all functions within a class generically, rather than explicitly stating it for every function.

I suppose it then becomes a kind of aspect, rather than a decorator and it does feel a bit odd, but was thinking for something like timing or auth it'd be pretty neat.

Answer 1

You could override the __getattr__ method. It's not actually attaching a decorator, but it lets you return a decorated method. You'd probably want to do something like this:

class Eggs(object):
    def __getattr__(self, attr):
        return decorate(getattr(self, `_` + attr))

There's some ugly recursion hiding in there that you'll want to protect against, but that's a start.

Answer 2

The cleanest way to do this, or to do other modifications to a class definition, is to define a metaclass.

Alternatively, just apply your decorator at the end of the class definition:

class Something:
   def foo(self): pass

for name, fn in inspect.getmembers(Something):
    if isinstance(fn, types.UnboundMethodType):
        setattr(Something, name, decorator(fn))

For Python 3 replace types.UnboundMethodType with types.FunctionType.

In practice of course you'll want to apply your decorator more selectively, and as soon as you want to decorate all but one method you'll discover that it is easier and more flexible just to use the decorator syntax in the traditional way.

Answer 3

Of course that the metaclasses are the most pythonic way to go when you want to modify the way that python creates the objects. Which can be done by overriding the __new__ method of your class. But there are some points around this problem (specially for python 3.X) that I'd like to mention:

1. types.FunctionType doesn't protect the special methods from being decorated, as they are function types. As a more general way you can just decorate the objects which their names are not started with double underscore (__). One other benefit of this method is that it also covers those objects that exist in namespace and starts with __ but are not function like __qualname__, __module__ , etc.

2. The namespace argument in __new__'s header doesn't contain class attributes within the __init__. The reason is that the __new__ executes before the __init__ (initializing).

3. It's not necessary to use a classmethod as the decorator, as in most of the times you import your decorator from another module.

4. If your class is contain a global item (out side of the __init__) for refusing of being decorated alongside checking if the name is not started with __ you can check the type with types.FunctionType to be sure that you're not decorating a non-function object.

---

Here is a sample metacalss that you can use:

class TheMeta(type):
    def __new__(cls, name, bases, namespace, **kwds):
        # if your decorator is a class method of the metaclass  use
        # `my_decorator = cls.my_decorator` in order to invoke the decorator.
        namespace = {k: v if k.startswith('__') else my_decorator(v) for k, v in namespace.items()}
        return type.__new__(cls, name, bases, namespace)

Demo:

def my_decorator(func):
        def wrapper(self, arg):
            # You can also use *args instead of (self, arg) and pass the *args
            # to the function in following call.
            return "the value {} gets modified!!".format(func(self, arg))
        return wrapper


class TheMeta(type):
    def __new__(cls, name, bases, namespace, **kwds):
        # my_decorator = cls.my_decorator (if the decorator is a classmethod)
        namespace = {k: v if k.startswith('__') else my_decorator(v) for k, v in namespace.items()}
        return type.__new__(cls, name, bases, namespace)


class MyClass(metaclass=TheMeta):
    # a = 10
    def __init__(self, *args, **kwargs):
        self.item = args[0]
        self.value = kwargs['value']

    def __getattr__(self, attr):
        return "This class hasn't provide the attribute {}.".format(attr)

    def myfunction_1(self, arg):
        return arg ** 2

    def myfunction_2(self, arg):
        return arg ** 3

myinstance = MyClass(1, 2, value=100)
print(myinstance.myfunction_1(5))
print(myinstance.myfunction_2(2))
print(myinstance.item)
print(myinstance.p)

Output:

the value 25 gets modified!!
the value 8 gets modified!!
1
This class hasn't provide the attribute p. # special method is not decorated.

For checking the 3rd item from the aforementioned notes you can uncomment the line a = 10 and do print(myinstance.a) and see the result then change the dictionary comprehension in __new__ as follows then see the result again:

namespace = {k: v if k.startswith('__') and not isinstance(v, types.FunctionType)\
             else my_decorator(v) for k, v in namespace.items()}

Answer 4

Everytime you think of changing class definition, you can either use the class decorator or metaclass. e.g. using metaclass

import types

class DecoMeta(type):
   def __new__(cls, name, bases, attrs):

      for attr_name, attr_value in attrs.iteritems():
         if isinstance(attr_value, types.FunctionType):
            attrs[attr_name] = cls.deco(attr_value)

      return super(DecoMeta, cls).__new__(cls, name, bases, attrs)

   @classmethod
   def deco(cls, func):
      def wrapper(*args, **kwargs):
         print "before",func.func_name
         result = func(*args, **kwargs)
         print "after",func.func_name
         return result
      return wrapper

class MyKlass(object):
   __metaclass__ = DecoMeta

   def func1(self): 
      pass

MyKlass().func1()

Output:

before func1
after func1

Note: it will not decorate staticmethod and classmethod