Python decorators in classes

What you’re wanting to do isn’t possible. Take, for instance, whether or not the code below looks valid:

class Test(object):

    def _decorator(self, foo):
        foo()

    def bar(self):
        pass
    bar = self._decorator(bar)

It, of course, isn’t valid since self isn’t defined at that point. The same goes for Test as it won’t be defined until the class itself is defined (which its in the process of). I’m showing you this code snippet because this is what your decorator snippet transforms into.

So, as you can see, accessing the instance in a decorator like that isn’t really possible since decorators are applied during the definition of whatever function/method they are attached to and not during instantiation.

If you need class-level access, try this:

class Test(object):

    @classmethod
    def _decorator(cls, foo):
        foo()

    def bar(self):
        pass
Test.bar = Test._decorator(Test.bar)

I found this question while researching a very similar problem. My solution is to split the problem into two parts. First, you need to capture the data that you want to associate with the class methods. In this case, handler_for will associate a Unix command with handler for that command’s output.

class OutputAnalysis(object):
    "analyze the output of diagnostic commands"
    def handler_for(name):
        "decorator to associate a function with a command"
        def wrapper(func):
            func.handler_for = name
            return func
        return wrapper
    # associate mount_p with 'mount_-p.txt'
    @handler_for('mount -p')
    def mount_p(self, slurped):
        pass

Now that we’ve associated some data with each class method, we need to gather that data and store it in a class attribute.

OutputAnalysis.cmd_handler = {}
for value in OutputAnalysis.__dict__.itervalues():
    try:
        OutputAnalysis.cmd_handler[value.handler_for] = value
    except AttributeError:
        pass

I use this type of decorator in some debugging situations, it allows overriding class properties by decorating, without having to find the calling function.

class myclass(object):
    def __init__(self):
        self.property = "HELLO"

    @adecorator(property="GOODBYE")
    def method(self):
        print self.property

Here is the decorator code

class adecorator (object):
    def __init__ (self, *args, **kwargs):
        # store arguments passed to the decorator
        self.args = args
        self.kwargs = kwargs

    def __call__(self, func):
        def newf(*args, **kwargs):

            #the 'self' for a method function is passed as args[0]
            slf = args[0]

            # replace and store the attributes
            saved = {}
            for k,v in self.kwargs.items():
                if hasattr(slf, k):
                    saved[k] = getattr(slf,k)
                    setattr(slf, k, v)

            # call the method
            ret = func(*args, **kwargs)

            #put things back
            for k,v in saved.items():
                setattr(slf, k, v)

            return ret
        newf.__doc__ = func.__doc__
        return newf 

Note: because I’ve used a class decorator you’ll need to use @adecorator() with the brackets on to decorate functions, even if you don’t pass any arguments to the decorator class constructor.

Decorators seem better suited to modify the functionality of an entire object (including function objects) versus the functionality of an object method which in general will depend on instance attributes. For example:

def mod_bar(cls):
    # returns modified class

    def decorate(fcn):
        # returns decorated function

        def new_fcn(self):
            print self.start_str
            print fcn(self)
            print self.end_str

        return new_fcn

    cls.bar = decorate(cls.bar)
    return cls

@mod_bar
class Test(object):
    def __init__(self):
        self.start_str = "starting dec"
        self.end_str = "ending dec" 

    def bar(self):
        return "bar"

The output is:

>>> import Test
>>> a = Test()
>>> a.bar()
starting dec
bar
ending dec

You can decorate the decorator:

import decorator

class Test(object):
    @decorator.decorator
    def _decorator(foo, self):
        foo(self)

    @_decorator
    def bar(self):
        pass

This is one way to access(and have used) self from inside a decorator defined inside the same class:

class Thing(object):
    def __init__(self, name):
        self.name = name

    def debug_name(function):
        def debug_wrapper(*args):
            self = args[0]
            print 'self.name = ' + self.name
            print 'running function {}()'.format(function.__name__)
            function(*args)
            print 'self.name = ' + self.name
        return debug_wrapper

    @debug_name
    def set_name(self, new_name):
        self.name = new_name

Output (tested on Python 2.7.10):

>>> a = Thing('A')
>>> a.name
'A'
>>> a.set_name('B')
self.name = A
running function set_name()
self.name = B
>>> a.name
'B'

The example above is silly, but it works.

import functools


class Example:

    def wrapper(func):
        @functools.wraps(func)
        def wrap(self, *args, **kwargs):
            print("inside wrap")
            return func(self, *args, **kwargs)
        return wrap

    @wrapper
    def method(self):
        print("METHOD")

    wrapper = staticmethod(wrapper)


e = Example()
e.method()

Here’s an expansion on Michael Speer’s answer to take it a few steps further:

An instance method decorator which takes arguments and acts on a function with arguments and a return value.

class Test(object):
    "Prints if x == y. Throws an error otherwise."
    def __init__(self, x):
        self.x = x

    def _outer_decorator(y):
        def _decorator(foo):
            def magic(self, *args, **kwargs) :
                print("start magic")
                if self.x == y:
                    return foo(self, *args, **kwargs)
                else:
                    raise ValueError("x ({}) != y ({})".format(self.x, y))
                print("end magic")
            return magic

        return _decorator

    @_outer_decorator(y=3)
    def bar(self, *args, **kwargs) :
        print("normal call")
        print("args: {}".format(args))
        print("kwargs: {}".format(kwargs))

        return 27

And then

In [2]:

    test = Test(3)
    test.bar(
        13,
        'Test',
        q=9,
        lollipop=[1,2,3]
    )
    ​
    start magic
    normal call
    args: (13, 'Test')
    kwargs: {'q': 9, 'lollipop': [1, 2, 3]}
Out[2]:
    27
In [3]:

    test = Test(4)
    test.bar(
        13,
        'Test',
        q=9,
        lollipop=[1,2,3]
    )
    ​
    start magic
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-3-576146b3d37e> in <module>()
          4     'Test',
          5     q=9,
    ----> 6     lollipop=[1,2,3]
          7 )

    <ipython-input-1-428f22ac6c9b> in magic(self, *args, **kwargs)
         11                     return foo(self, *args, **kwargs)
         12                 else:
    ---> 13                     raise ValueError("x ({}) != y ({})".format(self.x, y))
         14                 print("end magic")
         15             return magic

    ValueError: x (4) != y (3)

I have a Implementation of Decorators that Might Help

    import functools
    import datetime


    class Decorator(object):

        def __init__(self):
            pass


        def execution_time(func):

            @functools.wraps(func)
            def wrap(self, *args, **kwargs):

                """ Wrapper Function """

                start = datetime.datetime.now()
                Tem = func(self, *args, **kwargs)
                end = datetime.datetime.now()
                print("Exection Time:{}".format(end-start))
                return Tem

            return wrap


    class Test(Decorator):

        def __init__(self):
            self._MethodName = Test.funca.__name__

        @Decorator.execution_time
        def funca(self):
            print("Running Function : {}".format(self._MethodName))
            return True


    if __name__ == "__main__":
        obj = Test()
        data = obj.funca()
        print(data)

Declare in inner class.
This solution is pretty solid and recommended.

class Test(object):
    class Decorators(object):
    @staticmethod
    def decorator(foo):
        def magic(self, *args, **kwargs) :
            print("start magic")
            foo(self, *args, **kwargs)
            print("end magic")
        return magic

    @Decorators.decorator
    def bar( self ) :
        print("normal call")

test = Test()

test.bar()

The result:

>>> test = Test()
>>> test.bar()
start magic
normal call
end magic
>>> 

The simple way to do it.
All you need is to put the decorator method outside the class.
You can still use it inside.

def my_decorator(func):
    #this is the key line. There's the aditional self parameter
    def wrap(self, *args, **kwargs):
        # you can use self here as if you were inside the class
        return func(self, *args, **kwargs)
    return wrap

class Test(object):
    @my_decorator
    def bar(self):
        pass

Use a static method and include an additional parameter (self) in the inner function (wrapper) of the decorator.

import functools 

class Test:

@staticmethod
def _decorator(f):

    @functools.wraps(f)
    def _wrapper(self, *args, **kwargs):
        # do some serious decorating (incl. calls to self!)
        print(self)

        return f(self, *args, **kwargs)

    return _wrapper

@_decorator
def bar(self):
    return 42

For Python 3 and for the linters sake

def methoddecorator(deco: Callable[[Any, Callable], Callable]):
"""
Decorator to implement method decorators in the same class

Example of usage:

    class A:
        @methoddecorator
        def my_methods_deco(self, method):
            @wraps(method)
            def wrapper(this: 'A', *args, **kwargs):
                # do smth
                # N.B. for instance access use this, not self!
                return method(this, *args, **kwargs)
            return wrapper

        @my_methods_deco
        def my_method(self, a, b):
            ...

"""
@functools.wraps(deco)
def wrapped_deco(method):
    return deco(NotImplemented, method)
return wrapped_deco

One can use this uber-decorator to patch the classes.

BTW, this code does not support decorator parameters like @deco(param=...), but you have got the idea.