Python decorators in classes

Python Problem Overview

Can one write something like:

class Test(object):
    def _decorator(self, foo):
        foo()

    @self._decorator
    def bar(self):
        pass

This fails: self in @self is unknown

I also tried:

@Test._decorator(self)

which also fails: Test unknown

I would like to temporarily change some instance variables in the decorator and then run the decorated method, before changing them back.

Python Solutions

Solution 1 - Python

Would something like this do what you need?

class Test(object):
    def _decorator(foo):
        def magic( self ) :
            print "start magic"
            foo( self )
            print "end magic"
        return magic

    @_decorator
    def bar( self ) :
        print "normal call"

test = Test()

test.bar()

This avoids the call to self to access the decorator and leaves it hidden in the class namespace as a regular method.

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

edited to answer question in comments:

How to use the hidden decorator in another class

class Test(object):
    def _decorator(foo):
        def magic( self ) :
            print "start magic"
            foo( self )
            print "end magic"
        return magic

    @_decorator
    def bar( self ) :
        print "normal call"

    _decorator = staticmethod( _decorator )

class TestB( Test ):
    @Test._decorator
    def bar( self ):
        print "override bar in"
        super( TestB, self ).bar()
        print "override bar out"

print "Normal:"
test = Test()
test.bar()
print

print "Inherited:"
b = TestB()
b.bar()
print

Output:

Normal:
start magic
normal call
end magic

Inherited:
start magic
override bar in
start magic
normal call
end magic
override bar out
end magic

Solution 2 - Python

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)

Solution 3 - Python

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()

Solution 4 - Python

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.

Solution 5 - Python

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

Solution 6 - Python

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.

Solution 7 - Python

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)

Solution 8 - Python

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

Solution 9 - Python

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
>>>

Solution 10 - Python

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

Solution 11 - Python

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)

Solution 12 - Python

You can decorate the decorator:

import decorator

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

    @_decorator
    def bar(self):
        pass

Content Type	Original Author	Original Content on Stackoverflow
Question	hcvst	View Question on Stackoverflow
Solution 1 - Python	Michael Speer	View Answer on Stackoverflow
Solution 2 - Python	Evan Fosmark	View Answer on Stackoverflow
Solution 3 - Python	madjardi	View Answer on Stackoverflow
Solution 4 - Python	Gunnar Hermansson	View Answer on Stackoverflow
Solution 5 - Python	samwyse	View Answer on Stackoverflow
Solution 6 - Python	digitalacorn	View Answer on Stackoverflow
Solution 7 - Python	Oliver Evans	View Answer on Stackoverflow
Solution 8 - Python	mentatkgs	View Answer on Stackoverflow
Solution 9 - Python	Yarik Kovalchuk	View Answer on Stackoverflow
Solution 10 - Python	nicodjimenez	View Answer on Stackoverflow
Solution 11 - Python	Soumil Nitin Shah	View Answer on Stackoverflow
Solution 12 - Python	doep	View Answer on Stackoverflow