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How to add methods of class to a list inside the class with a decorator

I would like to update a "class-wide" list from a decorator that decorates the class' methods and adds each decorated method to that list.
This is what came to mind:
def add(meth: callable):
Spam.eggs.append(func)
return meth
class Spam:
eggs = []
#add
def meth(self):
pass
This won't work though because Spam hasn't finished defining itself when #add is reached, and thus add raises a NameError, as pointed out in the comments.
I also tried a class method:
class Spam:
eggs = []
#classmethod
def add(cls, meth: callable):
cls.eggs.append(meth)
return meth
#add
def meth(self):
pass
But this doesn't work either because when #add is reached, add is bound to the classmethod decorated instance, which is not callable.
Here is what I need this for:
I have a class with several methods that take one argument (besides self) that transform that object in such a way that these methods may be composed with one another. I want to decorate each of these in such a way that they're automatically added to a list in the class.
E.g.:
from typing import List
def transform_meth(meth: callable):
TextProcessor.transforms.add(meth)
return meth
class TextProcessor:
transforms: List[callable] = []
#transform_meth
def m1(self, text):
return text
#transform_meth
def m2(self, text):
return text
def transform(self, text):
for transform in self.transforms:
text = transform(text)
return text
I could add the methods in the list manually, but I find the decorator to be clearer since it is close to the definition of the method, and thus it is easier to remember to decorate a new method when defining it than adding it to the list manually.

Your current approach fails because when transform_meth is called, TextProcessor isn't bound to anything yet (or if it is, that object gets overwritten when the class statement completes).
The simple solution would be to define transform_meth inside the class statement, so that it could simply declare transforms as a nonlocal variable. However, that won't work because a class statement doesn't establish a new scope.
Instead, you can define a function that creates the decorator, which takes the desired list (at that point a just a name in the body of the class statement, not from any assumed scope). That function returns a closure over the list argument
so that you can append to it.
def make_decorator(lst):
# *This* will be the function bound to the name 'transform_meth'
def _(meth):
lst.append(meth)
return meth
return _
class TextProcessor:
transforms: List[callable] = []
transform_meth = make_decorator(transforms)
#transform_meth
def m1(self, text):
return text
#transform_meth
def m2(self, text):
return text
def transform(self, text):
for transform in self.transforms:
text = transform(text)
return text
del transform_meth # Not needed anymore, don't create a class attribute

Since the arg of each method is self you can append to the object instance like so:
from functools import wraps
def appender(f):
#wraps(f)
def func(*args, **kwargs):
if f not in args[0].transforms:
args[0].transforms.append(f)
return f(*args, **kwargs)
return func
class Foo(object):
def __init__(self):
self.transforms = []
#appender
def m1(self, arg1):
return arg1
#appender
def m2(self, arg1):
return arg1
def transform(self, text):
methods = [f for f in dir(self) if not f.startswith("__") and callable(getattr(self,f)) and f != 'transform']
for f in methods:
text = getattr(self,f)(text)
return text
f = Foo()
f.transform('your text here')
print(f.transforms)
Output:
[<function Foo.m1 at 0x1171e4e18>, <function Foo.m2 at 0x1171e4268>]

How to make python decorators work like a tag to make function calls "by tag"

I'm a python beginner and have an idea of the basic concept of decorators. I have worked on Python-Behave before and the framework allows you to insert a decorator and it would function like a tag. I'm trying to replicate this in a framework I'm currently building.
custom arg in the CLI
parser = argparse.ArgumentParser()
parser.add_argument('--type', help='foo help')
args = parser.parse_args() #args.type would receive the value
CLI
python run.py --type=A
Functions
#typeA
def foo_func():
print "type A ran"
#typeB
def bar_func():
print "type B ran"
Expected output
"type A ran"

Use decorator to store a reference to a function in dictionary. Use user input to retrieve function from dictionary. Call it afterwards.
REGISTER = {}
def register(name):
def wrapper(f):
print f, 'registered'
REGISTER[name] = f
return f
return wrapper
#register('A')
def foo():
print 'foo called'
#register('B')
def bar():
print 'bar called'
name = 'A' # or args.type
func_to_call = REGISTER[name]
func_to_call() # actual call is done here

Wrap your decorator up into a class style decorator to avoid having hanging variables and use a memoized factory function to create the tags.
import functools
class TagDecorator(object):
def __init__(self, tagName):
self.functions = []
self.tagName = tagName
def __str__(self):
return "<TagDecorator {tagName}>".format(tagName=self.tagName)
def __call__(self, f):
self.functions.append(f)
return f
def invoke(self, *args, **kwargs):
return [f(*args, **kwargs) for f in self.functions]
#functools.lru_cache(maxsize=None) # memoization
def get_func_tag(tagName):
return TagDecorator(tagName)
Now we create our tags:
tagA = get_func_tag("A")
tagB = get_func_tag("B")
#tagA
def funcA_1(*args, **kwargs):
print("A", args, kwargs)
# another way
#get_func_tag("A")
def funcA_2(*args, **kwargs):
print("A too", args, kwargs)
#tagB
def funcB_1():
print("B")
#tagB
def funcB_2():
print("B too")
# invoke all functions registered with tagA : passing arguments
tagA.invoke("hello", who="dolly")
# invoke all functions registered with tagA, another way.
get_func_tag("A").invoke()
# actually get_func_tag always returns the same instance
# for a given tagName, thanks to lru_cache
assert get_func_tag("A") == tagA
# Of course tagB can be invoked
tagB.invoke()
get_func_tag("B").invoke()
# but passing it args would be an error.
tagB.invoke("B") # TypeError: funcB_1() takes 0 positional arguments but 1 was given
Care has to be taken about the tagged functions' arguments. If for one same tag, you register signatures which vary, you will certainly have issues when invoking them. You can take care of this using the inspect.signature function. But real signature matching is a bit tricky.

Writing decorator for pytest test method

Assuming the following structure:
class SetupTestParam(object):
def setup_method(self, method):
self.foo = bar()
#pytest.fixture
def some_fixture():
self.baz = 'foobar'
I use SetupTestParam as a parent class for test classes.
class TestSomething(SetupTestParam):
def test_a_lot(self, some_fixture):
with self.baz as magic:
with magic.fooz as more_magic:
blah = more_magic.much_more_magic() # repetative bleh
... # not repetative code here
assert spam == 'something cool'
Now, writing tests gets repetitive (with statement usage) and I would like to write a decorator to reduce the number of code lines. But there is a problem with pytest and the function signature.
I found out library which should be helpful but I can't manage to get it to work.
I made a classmethod in my SetupTestParam class.
#classmethod
#decorator.decorator
def this_is_decorator(cls, f):
def wrapper(self, *args, **kw):
with self.baz as magic:
with magic.fooz as more_magic:
blah = more_magic.much_more_magic() # repetative bleh
return f(self, *args)
return wrapper
After I decorate the test_a_lot method, I receive the error TypeError: transaction_decorator() takes exactly 1 argument (2 given)
Can someone explain me please what am I doing wrong? (I assume there is a problem with self from the test method?)

Chaining decorators is not the simplest thing to do. One solution might be to separate the two decorators. Keep the classmethod but move decorator.decorator to the end:
#classmethod
def this_is_decorator(cls, f):
def wrapper(self, *args, **kw):
with self.baz as magic:
with magic.fooz as more_magic:
blah = more_magic.much_more_magic() # repetative bleh
return f(self, *args)
return decorator.decorator(wrapper, f)
Maybe this works for you.

After some tweaking and realizing that I need to pass a parameter to decorator I choosed to write it as a class:
class ThisIsDecorator(object):
def __init__(self, param):
self.param = param # Parameter may vary with the function being decorated
def __call__(self, fn):
wraps(fn) # [1]
def wrapper(fn, fn_self, *args): # [2] fn_self refers to original self param from function fn (test_a_lot) [2]
with fn_self.baz as fn_self.magic: # I pass magic to fn_self to make magic accesible in function fn (test_a_lot)
with fn_self.magic.fooz as more_magic:
blah = self.param.much_more_magic() # repetative bleh
return fn(fn_self, *args)
return decorator.decorator(wrapper, fn)
[1] I use wraps to have original fn __name__, __module__ and __doc__.
[2] Params passed to wrapper were self = <function test_a_lot at 0x24820c8> args = (<TestSomething object at 0x29c77d0>, None, None, None, None), kw = {} so I took out the args[0] as a fn_self.
Original version (without passing a parameter):
#classmethod
def this_is_decorator(cls, fn):
#wraps(fn)
def wrapper(fn, fn_self, *args):
with fn_self.baz as fn_self.magic:
with fn_self.magic.fooz as more_magic:
blah = more_magic.much_more_magic() # repetative bleh
return fn(fn_self, *args)
return decorator.decorator(wrapper,fn)
Thanks go to Mike Muller for pointing out the right direction.

Here's what happens in time order as this method is defined.
this_is_decorator is created (not called).
decorator.decorator(this_is_decorator) is called. This returns a new function which becomes this_is_decorator and has the same usage.
classmethod(this_is_decorator) is called, and the result of that is a classmethod that accepts (cls, f) and returns wrapper.
Later at runtime, a call to this_is_decorator will return wrapper.
But considering that this_is_decorator is a class method, it's not clear to me that this is what you want. I'm guessing that you may want something more like this:
from decorator import decorator
#decorator
def mydecorator(f):
def wrapper(cls, *args, **kw):
# ... logging, reporting, caching, whatever
return f(*args, **kw)
return wrapper
class MyClass(object):
#classmethod
#mydecorator
def myclsmethod(a, b, c):
# no cls or self value accepted here; this is a function not a method
# ...
Here your decorator is defined outside your class, because it's changing an ordinary function into a classmethod (and because you may want to use it in other places). The order of execution here is:
mydecorator is defined, not called.
decorator(mydecorator) is called, and the result becomes mydecorator.
Creation of MyClass starts.
myclsmethod is created. It is an ordinary function, not a method. There is a difference within the VM, so that you do not have to explicitly supply cls or self arguments to methods.
myclsmethod is passed to mydecorator (which has itself been decorated before) and the result (wrapper) is still a function not a method.
The result of mydecorator is passed to classmethod which returns an actual class method that is bound to MyClass.myclsmethod.
Definition of MyClass finishes.
Later when MyClass.myclsmethod(a, b, c) is called, wrapper executes, which then calls the original myclsmethod(a, b, c) function (which it knows as f) without supplying the cls argument.
Since you have an additional need to preserve the argument list exactly, so that even the names of the arguments are preserved in the decorated function, except with an extra initial argument cls, then you could implement mydecorator this way:
from decorator import decorator
from inspect import getargspec
#decorator
def mydecorator(func):
result = [None] # necessary so exec can "return" objects
namespace = {'f': func, 'result': result}
source = []
add = lambda indent, line: source.append(' ' * indent + line) # shorthand
arglist = ', '.join(getargspec(func).args) # this does not cover keyword or default args
add(0, 'def wrapper(cls, %s):' % (arglist,))
add(2, 'return f(%s)' % (arglist,))
add(0, 'result[0] = wrapper') # this is how to "return" something from exec
exec '\n'.join(source) in namespace
return result[0] # this is wrapper
It's kinda ugly, but this is the only way I know to dynamically set the argument list of a function based on data. If returning a lambda is OK, you could use eval instead of exec, which eliminates the need for an array that gets written into but is otherwise about the same.

Is it possible to replace a Python function/method decorator at runtime?

If I have a function :
#aDecorator
def myfunc1():
# do something here
if __name__ = "__main__":
# this will call the function and will use the decorator #aDecorator
myfunc1()
# now I want the #aDecorator to be replaced with the decorator #otherDecorator
# so that when this code executes, the function no longer goes through
# #aDecorator, but instead through #otherDecorator. How can I do this?
myfunc1()
Is it possible to replace a decorator at runtime?

As Miya mentioned, you can replace the decorator with another function any point before the interpreter gets to that function declaration. However, once the decorator is applied to the function, I don't think there is a way to dynamically replace the decorator with a different one. So for example:
#aDecorator
def myfunc1():
pass
# Oops! I didn't want that decorator after all!
myfunc1 = bDecorator(myfunc1)
Won't work, because myfunc1 is no longer the function you originally defined; it has already been wrapped. The best approach here is to manually apply the decorators, oldskool-style, i.e:
def myfunc1():
pass
myfunc2 = aDecorator(myfunc1)
myfunc3 = bDecorator(myfunc1)
Edit: Or, to be a little clearer,
def _tempFunc():
pass
myfunc1 = aDecorator(_tempFunc)
myfunc1()
myfunc1 = bDecorator(_tempFunc)
myfunc1()

I don't know if there's a way to "replace" a decorator once it has been applied, but I guess that probably there's not, because the function has already been changed.
You might, anyway, apply a decorator at runtime based on some condition:
#!/usr/bin/env python
class PrintCallInfo:
def __init__(self,f):
self.f = f
def __call__(self,*args,**kwargs):
print "-->",self.f.__name__,args,kwargs
r = self.f(*args,**kwargs)
print "<--",self.f.__name__,"returned: ",r
return r
# the condition to modify the function...
some_condition=True
def my_decorator(f):
if (some_condition): # modify the function
return PrintCallInfo(f)
else: # leave it as it is
return f
#my_decorator
def foo():
print "foo"
#my_decorator
def bar(s):
print "hello",s
return s
#my_decorator
def foobar(x=1,y=2):
print x,y
return x + y
foo()
bar("world")
foobar(y=5)

Here's a terrific recipe to get you started. Basically, the idea is to pass a class instance into the decorator. You can then set attributes on the class instance (make it a Borg if you like) and use that to control the behavior of the decorator itself.
Here's an example:
class Foo:
def __init__(self, do_apply):
self.do_apply = do_apply
def dec(foo):
def wrap(f):
def func(*args, **kwargs):
if foo.do_apply:
# Do something!
pass
return f(*args, **kwargs)
return func
return wrap
foo = Foo(False)
#dec(foo)
def bar(x):
return x
bar('bar')
foo.do_apply = True
# Decorator now active!
bar('baz')
Naturally, you can also incorporate the "decorator decorator" to preserve signatures, etc.

If you want to explicitely change the decorator, you might as well choose a more explicit approach instead of creating a decorated function:
deco1(myfunc1, arg1, arg2)
deco2(myfunc1, arg2, arg3)
deco1() and deco2() would apply the functionality your decorators provide and call myfunc1() with the arguments.

Sure - you can get the function object and do whatever you want with it:
# Bypass a decorator
import types
class decorator_test(object):
def __init__(self, f):
self.f = f
def __call__(self):
print "In decorator ... entering: ", self.f.__name__
self.f()
print "In decorator ... exiting: ", self.f.__name__
#decorator_test
def func1():
print "inside func1()"
print "\nCalling func1 with decorator..."
func1()
print "\nBypassing decorator..."
for value in func1.__dict__.values():
if isinstance(value, types.FunctionType) and value.func_name == "func1":
value.__call__()

I know it's an old thread, but I had fun doing this
def change_deco(name, deco, placeholder=' #'):
with open(name + '.py', 'r') as file:
lines = file.readlines()
for idx, string in enumerate(lines):
if placeholder in string and repr(placeholder) not in string:
lines[idx] = f' #{deco}\r\n'
exec(''.join(lines))
return locals()[name]

If the decorator is a function, just replace it.
aDecorator = otherDecorator

How do I make function decorators and chain them together?

How do I make two decorators in Python that would do the following?
#make_bold
#make_italic
def say():
return "Hello"
Calling say() should return:
"<b><i>Hello</i></b>"

If you are not into long explanations, see Paolo Bergantino’s answer.
Decorator Basics
Python’s functions are objects
To understand decorators, you must first understand that functions are objects in Python. This has important consequences. Let’s see why with a simple example :
def shout(word="yes"):
return word.capitalize()+"!"
print(shout())
# outputs : 'Yes!'
# As an object, you can assign the function to a variable like any other object
scream = shout
# Notice we don't use parentheses: we are not calling the function,
# we are putting the function "shout" into the variable "scream".
# It means you can then call "shout" from "scream":
print(scream())
# outputs : 'Yes!'
# More than that, it means you can remove the old name 'shout',
# and the function will still be accessible from 'scream'
del shout
try:
print(shout())
except NameError as e:
print(e)
#outputs: "name 'shout' is not defined"
print(scream())
# outputs: 'Yes!'
Keep this in mind. We’ll circle back to it shortly.
Another interesting property of Python functions is they can be defined inside another function!
def talk():
# You can define a function on the fly in "talk" ...
def whisper(word="yes"):
return word.lower()+"..."
# ... and use it right away!
print(whisper())
# You call "talk", that defines "whisper" EVERY TIME you call it, then
# "whisper" is called in "talk".
talk()
# outputs:
# "yes..."
# But "whisper" DOES NOT EXIST outside "talk":
try:
print(whisper())
except NameError as e:
print(e)
#outputs : "name 'whisper' is not defined"*
#Python's functions are objects
Functions references
Okay, still here? Now the fun part...
You’ve seen that functions are objects. Therefore, functions:
can be assigned to a variable
can be defined in another function
That means that a function can return another function.
def getTalk(kind="shout"):
# We define functions on the fly
def shout(word="yes"):
return word.capitalize()+"!"
def whisper(word="yes") :
return word.lower()+"..."
# Then we return one of them
if kind == "shout":
# We don't use "()", we are not calling the function,
# we are returning the function object
return shout
else:
return whisper
# How do you use this strange beast?
# Get the function and assign it to a variable
talk = getTalk()
# You can see that "talk" is here a function object:
print(talk)
#outputs : <function shout at 0xb7ea817c>
# The object is the one returned by the function:
print(talk())
#outputs : Yes!
# And you can even use it directly if you feel wild:
print(getTalk("whisper")())
#outputs : yes...
There’s more!
If you can return a function, you can pass one as a parameter:
def doSomethingBefore(func):
print("I do something before then I call the function you gave me")
print(func())
doSomethingBefore(scream)
#outputs:
#I do something before then I call the function you gave me
#Yes!
Well, you just have everything needed to understand decorators. You see, decorators are “wrappers”, which means that they let you execute code before and after the function they decorate without modifying the function itself.
Handcrafted decorators
How you’d do it manually:
# A decorator is a function that expects ANOTHER function as parameter
def my_shiny_new_decorator(a_function_to_decorate):
# Inside, the decorator defines a function on the fly: the wrapper.
# This function is going to be wrapped around the original function
# so it can execute code before and after it.
def the_wrapper_around_the_original_function():
# Put here the code you want to be executed BEFORE the original function is called
print("Before the function runs")
# Call the function here (using parentheses)
a_function_to_decorate()
# Put here the code you want to be executed AFTER the original function is called
print("After the function runs")
# At this point, "a_function_to_decorate" HAS NEVER BEEN EXECUTED.
# We return the wrapper function we have just created.
# The wrapper contains the function and the code to execute before and after. It’s ready to use!
return the_wrapper_around_the_original_function
# Now imagine you create a function you don't want to ever touch again.
def a_stand_alone_function():
print("I am a stand alone function, don't you dare modify me")
a_stand_alone_function()
#outputs: I am a stand alone function, don't you dare modify me
# Well, you can decorate it to extend its behavior.
# Just pass it to the decorator, it will wrap it dynamically in
# any code you want and return you a new function ready to be used:
a_stand_alone_function_decorated = my_shiny_new_decorator(a_stand_alone_function)
a_stand_alone_function_decorated()
#outputs:
#Before the function runs
#I am a stand alone function, don't you dare modify me
#After the function runs
Now, you probably want that every time you call a_stand_alone_function, a_stand_alone_function_decorated is called instead. That’s easy, just overwrite a_stand_alone_function with the function returned by my_shiny_new_decorator:
a_stand_alone_function = my_shiny_new_decorator(a_stand_alone_function)
a_stand_alone_function()
#outputs:
#Before the function runs
#I am a stand alone function, don't you dare modify me
#After the function runs
# That’s EXACTLY what decorators do!
Decorators demystified
The previous example, using the decorator syntax:
#my_shiny_new_decorator
def another_stand_alone_function():
print("Leave me alone")
another_stand_alone_function()
#outputs:
#Before the function runs
#Leave me alone
#After the function runs
Yes, that’s all, it’s that simple. #decorator is just a shortcut to:
another_stand_alone_function = my_shiny_new_decorator(another_stand_alone_function)
Decorators are just a pythonic variant of the decorator design pattern. There are several classic design patterns embedded in Python to ease development (like iterators).
Of course, you can accumulate decorators:
def bread(func):
def wrapper():
print("</''''''\>")
func()
print("<\______/>")
return wrapper
def ingredients(func):
def wrapper():
print("#tomatoes#")
func()
print("~salad~")
return wrapper
def sandwich(food="--ham--"):
print(food)
sandwich()
#outputs: --ham--
sandwich = bread(ingredients(sandwich))
sandwich()
#outputs:
#</''''''\>
# #tomatoes#
# --ham--
# ~salad~
#<\______/>
Using the Python decorator syntax:
#bread
#ingredients
def sandwich(food="--ham--"):
print(food)
sandwich()
#outputs:
#</''''''\>
# #tomatoes#
# --ham--
# ~salad~
#<\______/>
The order you set the decorators MATTERS:
#ingredients
#bread
def strange_sandwich(food="--ham--"):
print(food)
strange_sandwich()
#outputs:
##tomatoes#
#</''''''\>
# --ham--
#<\______/>
# ~salad~
Now: to answer the question...
As a conclusion, you can easily see how to answer the question:
# The decorator to make it bold
def makebold(fn):
# The new function the decorator returns
def wrapper():
# Insertion of some code before and after
return "<b>" + fn() + "</b>"
return wrapper
# The decorator to make it italic
def makeitalic(fn):
# The new function the decorator returns
def wrapper():
# Insertion of some code before and after
return "<i>" + fn() + "</i>"
return wrapper
#makebold
#makeitalic
def say():
return "hello"
print(say())
#outputs: <b><i>hello</i></b>
# This is the exact equivalent to
def say():
return "hello"
say = makebold(makeitalic(say))
print(say())
#outputs: <b><i>hello</i></b>
You can now just leave happy, or burn your brain a little bit more and see advanced uses of decorators.
Taking decorators to the next level
Passing arguments to the decorated function
# It’s not black magic, you just have to let the wrapper
# pass the argument:
def a_decorator_passing_arguments(function_to_decorate):
def a_wrapper_accepting_arguments(arg1, arg2):
print("I got args! Look: {0}, {1}".format(arg1, arg2))
function_to_decorate(arg1, arg2)
return a_wrapper_accepting_arguments
# Since when you are calling the function returned by the decorator, you are
# calling the wrapper, passing arguments to the wrapper will let it pass them to
# the decorated function
#a_decorator_passing_arguments
def print_full_name(first_name, last_name):
print("My name is {0} {1}".format(first_name, last_name))
print_full_name("Peter", "Venkman")
# outputs:
#I got args! Look: Peter Venkman
#My name is Peter Venkman
Decorating methods
One nifty thing about Python is that methods and functions are really the same. The only difference is that methods expect that their first argument is a reference to the current object (self).
That means you can build a decorator for methods the same way! Just remember to take self into consideration:
def method_friendly_decorator(method_to_decorate):
def wrapper(self, lie):
lie = lie - 3 # very friendly, decrease age even more :-)
return method_to_decorate(self, lie)
return wrapper
class Lucy(object):
def __init__(self):
self.age = 32
#method_friendly_decorator
def sayYourAge(self, lie):
print("I am {0}, what did you think?".format(self.age + lie))
l = Lucy()
l.sayYourAge(-3)
#outputs: I am 26, what did you think?
If you’re making general-purpose decorator--one you’ll apply to any function or method, no matter its arguments--then just use *args, **kwargs:
def a_decorator_passing_arbitrary_arguments(function_to_decorate):
# The wrapper accepts any arguments
def a_wrapper_accepting_arbitrary_arguments(*args, **kwargs):
print("Do I have args?:")
print(args)
print(kwargs)
# Then you unpack the arguments, here *args, **kwargs
# If you are not familiar with unpacking, check:
# http://www.saltycrane.com/blog/2008/01/how-to-use-args-and-kwargs-in-python/
function_to_decorate(*args, **kwargs)
return a_wrapper_accepting_arbitrary_arguments
#a_decorator_passing_arbitrary_arguments
def function_with_no_argument():
print("Python is cool, no argument here.")
function_with_no_argument()
#outputs
#Do I have args?:
#()
#{}
#Python is cool, no argument here.
#a_decorator_passing_arbitrary_arguments
def function_with_arguments(a, b, c):
print(a, b, c)
function_with_arguments(1,2,3)
#outputs
#Do I have args?:
#(1, 2, 3)
#{}
#1 2 3
#a_decorator_passing_arbitrary_arguments
def function_with_named_arguments(a, b, c, platypus="Why not ?"):
print("Do {0}, {1} and {2} like platypus? {3}".format(a, b, c, platypus))
function_with_named_arguments("Bill", "Linus", "Steve", platypus="Indeed!")
#outputs
#Do I have args ? :
#('Bill', 'Linus', 'Steve')
#{'platypus': 'Indeed!'}
#Do Bill, Linus and Steve like platypus? Indeed!
class Mary(object):
def __init__(self):
self.age = 31
#a_decorator_passing_arbitrary_arguments
def sayYourAge(self, lie=-3): # You can now add a default value
print("I am {0}, what did you think?".format(self.age + lie))
m = Mary()
m.sayYourAge()
#outputs
# Do I have args?:
#(<__main__.Mary object at 0xb7d303ac>,)
#{}
#I am 28, what did you think?
Passing arguments to the decorator
Great, now what would you say about passing arguments to the decorator itself?
This can get somewhat twisted, since a decorator must accept a function as an argument. Therefore, you cannot pass the decorated function’s arguments directly to the decorator.
Before rushing to the solution, let’s write a little reminder:
# Decorators are ORDINARY functions
def my_decorator(func):
print("I am an ordinary function")
def wrapper():
print("I am function returned by the decorator")
func()
return wrapper
# Therefore, you can call it without any "#"
def lazy_function():
print("zzzzzzzz")
decorated_function = my_decorator(lazy_function)
#outputs: I am an ordinary function
# It outputs "I am an ordinary function", because that’s just what you do:
# calling a function. Nothing magic.
#my_decorator
def lazy_function():
print("zzzzzzzz")
#outputs: I am an ordinary function
It’s exactly the same. "my_decorator" is called. So when you #my_decorator, you are telling Python to call the function 'labelled by the variable "my_decorator"'.
This is important! The label you give can point directly to the decorator—or not.
Let’s get evil. ☺
def decorator_maker():
print("I make decorators! I am executed only once: "
"when you make me create a decorator.")
def my_decorator(func):
print("I am a decorator! I am executed only when you decorate a function.")
def wrapped():
print("I am the wrapper around the decorated function. "
"I am called when you call the decorated function. "
"As the wrapper, I return the RESULT of the decorated function.")
return func()
print("As the decorator, I return the wrapped function.")
return wrapped
print("As a decorator maker, I return a decorator")
return my_decorator
# Let’s create a decorator. It’s just a new function after all.
new_decorator = decorator_maker()
#outputs:
#I make decorators! I am executed only once: when you make me create a decorator.
#As a decorator maker, I return a decorator
# Then we decorate the function
def decorated_function():
print("I am the decorated function.")
decorated_function = new_decorator(decorated_function)
#outputs:
#I am a decorator! I am executed only when you decorate a function.
#As the decorator, I return the wrapped function
# Let’s call the function:
decorated_function()
#outputs:
#I am the wrapper around the decorated function. I am called when you call the decorated function.
#As the wrapper, I return the RESULT of the decorated function.
#I am the decorated function.
No surprise here.
Let’s do EXACTLY the same thing, but skip all the pesky intermediate variables:
def decorated_function():
print("I am the decorated function.")
decorated_function = decorator_maker()(decorated_function)
#outputs:
#I make decorators! I am executed only once: when you make me create a decorator.
#As a decorator maker, I return a decorator
#I am a decorator! I am executed only when you decorate a function.
#As the decorator, I return the wrapped function.
# Finally:
decorated_function()
#outputs:
#I am the wrapper around the decorated function. I am called when you call the decorated function.
#As the wrapper, I return the RESULT of the decorated function.
#I am the decorated function.
Let’s make it even shorter:
#decorator_maker()
def decorated_function():
print("I am the decorated function.")
#outputs:
#I make decorators! I am executed only once: when you make me create a decorator.
#As a decorator maker, I return a decorator
#I am a decorator! I am executed only when you decorate a function.
#As the decorator, I return the wrapped function.
#Eventually:
decorated_function()
#outputs:
#I am the wrapper around the decorated function. I am called when you call the decorated function.
#As the wrapper, I return the RESULT of the decorated function.
#I am the decorated function.
Hey, did you see that? We used a function call with the "#" syntax! :-)
So, back to decorators with arguments. If we can use functions to generate the decorator on the fly, we can pass arguments to that function, right?
def decorator_maker_with_arguments(decorator_arg1, decorator_arg2):
print("I make decorators! And I accept arguments: {0}, {1}".format(decorator_arg1, decorator_arg2))
def my_decorator(func):
# The ability to pass arguments here is a gift from closures.
# If you are not comfortable with closures, you can assume it’s ok,
# or read: https://stackoverflow.com/questions/13857/can-you-explain-closures-as-they-relate-to-python
print("I am the decorator. Somehow you passed me arguments: {0}, {1}".format(decorator_arg1, decorator_arg2))
# Don't confuse decorator arguments and function arguments!
def wrapped(function_arg1, function_arg2) :
print("I am the wrapper around the decorated function.\n"
"I can access all the variables\n"
"\t- from the decorator: {0} {1}\n"
"\t- from the function call: {2} {3}\n"
"Then I can pass them to the decorated function"
.format(decorator_arg1, decorator_arg2,
function_arg1, function_arg2))
return func(function_arg1, function_arg2)
return wrapped
return my_decorator
#decorator_maker_with_arguments("Leonard", "Sheldon")
def decorated_function_with_arguments(function_arg1, function_arg2):
print("I am the decorated function and only knows about my arguments: {0}"
" {1}".format(function_arg1, function_arg2))
decorated_function_with_arguments("Rajesh", "Howard")
#outputs:
#I make decorators! And I accept arguments: Leonard Sheldon
#I am the decorator. Somehow you passed me arguments: Leonard Sheldon
#I am the wrapper around the decorated function.
#I can access all the variables
# - from the decorator: Leonard Sheldon
# - from the function call: Rajesh Howard
#Then I can pass them to the decorated function
#I am the decorated function and only knows about my arguments: Rajesh Howard
Here it is: a decorator with arguments. Arguments can be set as variable:
c1 = "Penny"
c2 = "Leslie"
#decorator_maker_with_arguments("Leonard", c1)
def decorated_function_with_arguments(function_arg1, function_arg2):
print("I am the decorated function and only knows about my arguments:"
" {0} {1}".format(function_arg1, function_arg2))
decorated_function_with_arguments(c2, "Howard")
#outputs:
#I make decorators! And I accept arguments: Leonard Penny
#I am the decorator. Somehow you passed me arguments: Leonard Penny
#I am the wrapper around the decorated function.
#I can access all the variables
# - from the decorator: Leonard Penny
# - from the function call: Leslie Howard
#Then I can pass them to the decorated function
#I am the decorated function and only know about my arguments: Leslie Howard
As you can see, you can pass arguments to the decorator like any function using this trick. You can even use *args, **kwargs if you wish. But remember decorators are called only once. Just when Python imports the script. You can't dynamically set the arguments afterwards. When you do "import x", the function is already decorated, so you can't
change anything.
Let’s practice: decorating a decorator
Okay, as a bonus, I'll give you a snippet to make any decorator accept generically any argument. After all, in order to accept arguments, we created our decorator using another function.
We wrapped the decorator.
Anything else we saw recently that wrapped function?
Oh yes, decorators!
Let’s have some fun and write a decorator for the decorators:
def decorator_with_args(decorator_to_enhance):
"""
This function is supposed to be used as a decorator.
It must decorate an other function, that is intended to be used as a decorator.
Take a cup of coffee.
It will allow any decorator to accept an arbitrary number of arguments,
saving you the headache to remember how to do that every time.
"""
# We use the same trick we did to pass arguments
def decorator_maker(*args, **kwargs):
# We create on the fly a decorator that accepts only a function
# but keeps the passed arguments from the maker.
def decorator_wrapper(func):
# We return the result of the original decorator, which, after all,
# IS JUST AN ORDINARY FUNCTION (which returns a function).
# Only pitfall: the decorator must have this specific signature or it won't work:
return decorator_to_enhance(func, *args, **kwargs)
return decorator_wrapper
return decorator_maker
It can be used as follows:
# You create the function you will use as a decorator. And stick a decorator on it :-)
# Don't forget, the signature is "decorator(func, *args, **kwargs)"
#decorator_with_args
def decorated_decorator(func, *args, **kwargs):
def wrapper(function_arg1, function_arg2):
print("Decorated with {0} {1}".format(args, kwargs))
return func(function_arg1, function_arg2)
return wrapper
# Then you decorate the functions you wish with your brand new decorated decorator.
#decorated_decorator(42, 404, 1024)
def decorated_function(function_arg1, function_arg2):
print("Hello {0} {1}".format(function_arg1, function_arg2))
decorated_function("Universe and", "everything")
#outputs:
#Decorated with (42, 404, 1024) {}
#Hello Universe and everything
# Whoooot!
I know, the last time you had this feeling, it was after listening a guy saying: "before understanding recursion, you must first understand recursion". But now, don't you feel good about mastering this?
Best practices: decorators
Decorators were introduced in Python 2.4, so be sure your code will be run on >= 2.4.
Decorators slow down the function call. Keep that in mind.
You cannot un-decorate a function. (There are hacks to create decorators that can be removed, but nobody uses them.) So once a function is decorated, it’s decorated for all the code.
Decorators wrap functions, which can make them hard to debug. (This gets better from Python >= 2.5; see below.)
The functools module was introduced in Python 2.5. It includes the function functools.wraps(), which copies the name, module, and docstring of the decorated function to its wrapper.
(Fun fact: functools.wraps() is a decorator! ☺)
# For debugging, the stacktrace prints you the function __name__
def foo():
print("foo")
print(foo.__name__)
#outputs: foo
# With a decorator, it gets messy
def bar(func):
def wrapper():
print("bar")
return func()
return wrapper
#bar
def foo():
print("foo")
print(foo.__name__)
#outputs: wrapper
# "functools" can help for that
import functools
def bar(func):
# We say that "wrapper", is wrapping "func"
# and the magic begins
#functools.wraps(func)
def wrapper():
print("bar")
return func()
return wrapper
#bar
def foo():
print("foo")
print(foo.__name__)
#outputs: foo
How can the decorators be useful?
Now the big question: What can I use decorators for?
Seem cool and powerful, but a practical example would be great. Well, there are 1000 possibilities. Classic uses are extending a function behavior from an external lib (you can't modify it), or for debugging (you don't want to modify it because it’s temporary).
You can use them to extend several functions in a DRY’s way, like so:
def benchmark(func):
"""
A decorator that prints the time a function takes
to execute.
"""
import time
def wrapper(*args, **kwargs):
t = time.clock()
res = func(*args, **kwargs)
print("{0} {1}".format(func.__name__, time.clock()-t))
return res
return wrapper
def logging(func):
"""
A decorator that logs the activity of the script.
(it actually just prints it, but it could be logging!)
"""
def wrapper(*args, **kwargs):
res = func(*args, **kwargs)
print("{0} {1} {2}".format(func.__name__, args, kwargs))
return res
return wrapper
def counter(func):
"""
A decorator that counts and prints the number of times a function has been executed
"""
def wrapper(*args, **kwargs):
wrapper.count = wrapper.count + 1
res = func(*args, **kwargs)
print("{0} has been used: {1}x".format(func.__name__, wrapper.count))
return res
wrapper.count = 0
return wrapper
#counter
#benchmark
#logging
def reverse_string(string):
return str(reversed(string))
print(reverse_string("Able was I ere I saw Elba"))
print(reverse_string("A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal: Panama!"))
#outputs:
#reverse_string ('Able was I ere I saw Elba',) {}
#wrapper 0.0
#wrapper has been used: 1x
#ablE was I ere I saw elbA
#reverse_string ('A man, a plan, a canoe, pasta, heros, rajahs, a coloratura, maps, snipe, percale, macaroni, a gag, a banana bag, a tan, a tag, a banana bag again (or a camel), a crepe, pins, Spam, a rut, a Rolo, cash, a jar, sore hats, a peon, a canal: Panama!',) {}
#wrapper 0.0
#wrapper has been used: 2x
#!amanaP :lanac a ,noep a ,stah eros ,raj a ,hsac ,oloR a ,tur a ,mapS ,snip ,eperc a ,)lemac a ro( niaga gab ananab a ,gat a ,nat a ,gab ananab a ,gag a ,inoracam ,elacrep ,epins ,spam ,arutaroloc a ,shajar ,soreh ,atsap ,eonac a ,nalp a ,nam A
Of course the good thing with decorators is that you can use them right away on almost anything without rewriting. DRY, I said:
#counter
#benchmark
#logging
def get_random_futurama_quote():
from urllib import urlopen
result = urlopen("http://subfusion.net/cgi-bin/quote.pl?quote=futurama").read()
try:
value = result.split("<br><b><hr><br>")[1].split("<br><br><hr>")[0]
return value.strip()
except:
return "No, I'm ... doesn't!"
print(get_random_futurama_quote())
print(get_random_futurama_quote())
#outputs:
#get_random_futurama_quote () {}
#wrapper 0.02
#wrapper has been used: 1x
#The laws of science be a harsh mistress.
#get_random_futurama_quote () {}
#wrapper 0.01
#wrapper has been used: 2x
#Curse you, merciful Poseidon!
Python itself provides several decorators: property, staticmethod, etc.
Django uses decorators to manage caching and view permissions.
Twisted to fake inlining asynchronous functions calls.
This really is a large playground.

Check out the documentation to see how decorators work. Here is what you asked for:
from functools import wraps
def makebold(fn):
#wraps(fn)
def wrapper(*args, **kwargs):
return "<b>" + fn(*args, **kwargs) + "</b>"
return wrapper
def makeitalic(fn):
#wraps(fn)
def wrapper(*args, **kwargs):
return "<i>" + fn(*args, **kwargs) + "</i>"
return wrapper
#makebold
#makeitalic
def hello():
return "hello world"
#makebold
#makeitalic
def log(s):
return s
print hello() # returns "<b><i>hello world</i></b>"
print hello.__name__ # with functools.wraps() this returns "hello"
print log('hello') # returns "<b><i>hello</i></b>"

Alternatively, you could write a factory function which return a decorator which wraps the return value of the decorated function in a tag passed to the factory function. For example:
from functools import wraps
def wrap_in_tag(tag):
def factory(func):
#wraps(func)
def decorator():
return '<%(tag)s>%(rv)s</%(tag)s>' % (
{'tag': tag, 'rv': func()})
return decorator
return factory
This enables you to write:
#wrap_in_tag('b')
#wrap_in_tag('i')
def say():
return 'hello'
or
makebold = wrap_in_tag('b')
makeitalic = wrap_in_tag('i')
#makebold
#makeitalic
def say():
return 'hello'
Personally I would have written the decorator somewhat differently:
from functools import wraps
def wrap_in_tag(tag):
def factory(func):
#wraps(func)
def decorator(val):
return func('<%(tag)s>%(val)s</%(tag)s>' %
{'tag': tag, 'val': val})
return decorator
return factory
which would yield:
#wrap_in_tag('b')
#wrap_in_tag('i')
def say(val):
return val
say('hello')
Don't forget the construction for which decorator syntax is a shorthand:
say = wrap_in_tag('b')(wrap_in_tag('i')(say)))

Decorators are just syntactical sugar.
This
#decorator
def func():
...
expands to
def func():
...
func = decorator(func)

And of course you can return lambdas as well from a decorator function:
def makebold(f):
return lambda: "<b>" + f() + "</b>"
def makeitalic(f):
return lambda: "<i>" + f() + "</i>"
#makebold
#makeitalic
def say():
return "Hello"
print say()

Python decorators add extra functionality to another function
An italics decorator could be like
def makeitalic(fn):
def newFunc():
return "<i>" + fn() + "</i>"
return newFunc
Note that a function is defined inside a function.
What it basically does is replace a function with the newly defined one. For example, I have this class
class foo:
def bar(self):
print "hi"
def foobar(self):
print "hi again"
Now say, I want both functions to print "---" after and before they are done.
I could add a print "---" before and after each print statement.
But because I don't like repeating myself, I will make a decorator
def addDashes(fn): # notice it takes a function as an argument
def newFunction(self): # define a new function
print "---"
fn(self) # call the original function
print "---"
return newFunction
# Return the newly defined function - it will "replace" the original
So now I can change my class to
class foo:
#addDashes
def bar(self):
print "hi"
#addDashes
def foobar(self):
print "hi again"
For more on decorators, check
http://www.ibm.com/developerworks/linux/library/l-cpdecor.html

You could make two separate decorators that do what you want as illustrated directly below. Note the use of *args, **kwargs in the declaration of the wrapped() function which supports the decorated function having multiple arguments (which isn't really necessary for the example say() function, but is included for generality).
For similar reasons, the functools.wraps decorator is used to change the meta attributes of the wrapped function to be those of the one being decorated. This makes error messages and embedded function documentation (func.__doc__) be those of the decorated function instead of wrapped()'s.
from functools import wraps
def makebold(fn):
#wraps(fn)
def wrapped(*args, **kwargs):
return "<b>" + fn(*args, **kwargs) + "</b>"
return wrapped
def makeitalic(fn):
#wraps(fn)
def wrapped(*args, **kwargs):
return "<i>" + fn(*args, **kwargs) + "</i>"
return wrapped
#makebold
#makeitalic
def say():
return 'Hello'
print(say()) # -> <b><i>Hello</i></b>
Refinements
As you can see there's a lot of duplicate code in these two decorators. Given this similarity it would be better for you to instead make a generic one that was actually a decorator factory—in other words, a decorator function that makes other decorators. That way there would be less code repetition—and allow the DRY principle to be followed.
def html_deco(tag):
def decorator(fn):
#wraps(fn)
def wrapped(*args, **kwargs):
return '<%s>' % tag + fn(*args, **kwargs) + '</%s>' % tag
return wrapped
return decorator
#html_deco('b')
#html_deco('i')
def greet(whom=''):
return 'Hello' + (' ' + whom) if whom else ''
print(greet('world')) # -> <b><i>Hello world</i></b>
To make the code more readable, you can assign a more descriptive name to the factory-generated decorators:
makebold = html_deco('b')
makeitalic = html_deco('i')
#makebold
#makeitalic
def greet(whom=''):
return 'Hello' + (' ' + whom) if whom else ''
print(greet('world')) # -> <b><i>Hello world</i></b>
or even combine them like this:
makebolditalic = lambda fn: makebold(makeitalic(fn))
#makebolditalic
def greet(whom=''):
return 'Hello' + (' ' + whom) if whom else ''
print(greet('world')) # -> <b><i>Hello world</i></b>
Efficiency
While the above examples do all work, the code generated involves a fair amount of overhead in the form of extraneous function calls when multiple decorators are applied at once. This may not matter, depending the exact usage (which might be I/O-bound, for instance).
If speed of the decorated function is important, the overhead can be kept to a single extra function call by writing a slightly different decorator factory-function which implements adding all the tags at once, so it can generate code that avoids the addtional function calls incurred by using separate decorators for each tag.
This requires more code in the decorator itself, but this only runs when it's being applied to function definitions, not later when they themselves are called. This also applies when creating more readable names by using lambda functions as previously illustrated. Sample:
def multi_html_deco(*tags):
start_tags, end_tags = [], []
for tag in tags:
start_tags.append('<%s>' % tag)
end_tags.append('</%s>' % tag)
start_tags = ''.join(start_tags)
end_tags = ''.join(reversed(end_tags))
def decorator(fn):
#wraps(fn)
def wrapped(*args, **kwargs):
return start_tags + fn(*args, **kwargs) + end_tags
return wrapped
return decorator
makebolditalic = multi_html_deco('b', 'i')
#makebolditalic
def greet(whom=''):
return 'Hello' + (' ' + whom) if whom else ''
print(greet('world')) # -> <b><i>Hello world</i></b>

Another way of doing the same thing:
class bol(object):
def __init__(self, f):
self.f = f
def __call__(self):
return "<b>{}</b>".format(self.f())
class ita(object):
def __init__(self, f):
self.f = f
def __call__(self):
return "<i>{}</i>".format(self.f())
#bol
#ita
def sayhi():
return 'hi'
Or, more flexibly:
class sty(object):
def __init__(self, tag):
self.tag = tag
def __call__(self, f):
def newf():
return "<{tag}>{res}</{tag}>".format(res=f(), tag=self.tag)
return newf
#sty('b')
#sty('i')
def sayhi():
return 'hi'

How can I make two decorators in Python that would do the following?
You want the following function, when called:
#makebold
#makeitalic
def say():
return "Hello"
To return:
<b><i>Hello</i></b>
Simple solution
To most simply do this, make decorators that return lambdas (anonymous functions) that close over the function (closures) and call it:
def makeitalic(fn):
return lambda: '<i>' + fn() + '</i>'
def makebold(fn):
return lambda: '<b>' + fn() + '</b>'
Now use them as desired:
#makebold
#makeitalic
def say():
return 'Hello'
and now:
>>> say()
'<b><i>Hello</i></b>'
Problems with the simple solution
But we seem to have nearly lost the original function.
>>> say
<function <lambda> at 0x4ACFA070>
To find it, we'd need to dig into the closure of each lambda, one of which is buried in the other:
>>> say.__closure__[0].cell_contents
<function <lambda> at 0x4ACFA030>
>>> say.__closure__[0].cell_contents.__closure__[0].cell_contents
<function say at 0x4ACFA730>
So if we put documentation on this function, or wanted to be able to decorate functions that take more than one argument, or we just wanted to know what function we were looking at in a debugging session, we need to do a bit more with our wrapper.
Full featured solution - overcoming most of these problems
We have the decorator wraps from the functools module in the standard library!
from functools import wraps
def makeitalic(fn):
# must assign/update attributes from wrapped function to wrapper
# __module__, __name__, __doc__, and __dict__ by default
#wraps(fn) # explicitly give function whose attributes it is applying
def wrapped(*args, **kwargs):
return '<i>' + fn(*args, **kwargs) + '</i>'
return wrapped
def makebold(fn):
#wraps(fn)
def wrapped(*args, **kwargs):
return '<b>' + fn(*args, **kwargs) + '</b>'
return wrapped
It is unfortunate that there's still some boilerplate, but this is about as simple as we can make it.
In Python 3, you also get __qualname__ and __annotations__ assigned by default.
So now:
#makebold
#makeitalic
def say():
"""This function returns a bolded, italicized 'hello'"""
return 'Hello'
And now:
>>> say
<function say at 0x14BB8F70>
>>> help(say)
Help on function say in module __main__:
say(*args, **kwargs)
This function returns a bolded, italicized 'hello'
Conclusion
So we see that wraps makes the wrapping function do almost everything except tell us exactly what the function takes as arguments.
There are other modules that may attempt to tackle the problem, but the solution is not yet in the standard library.

A decorator takes the function definition and creates a new function that executes this function and transforms the result.
#deco
def do():
...
is equivalent to:
do = deco(do)
Example:
def deco(func):
def inner(letter):
return func(letter).upper() #upper
return inner
This
#deco
def do(number):
return chr(number) # number to letter
is equivalent to this
def do2(number):
return chr(number)
do2 = deco(do2)
65 <=> 'a'
print(do(65))
print(do2(65))
>>> B
>>> B
To understand the decorator, it is important to notice, that decorator created a new function do which is inner that executes function and transforms the result.

This answer has long been answered, but I thought I would share my Decorator class which makes writing new decorators easy and compact.
from abc import ABCMeta, abstractclassmethod
class Decorator(metaclass=ABCMeta):
""" Acts as a base class for all decorators """
def __init__(self):
self.method = None
def __call__(self, method):
self.method = method
return self.call
#abstractclassmethod
def call(self, *args, **kwargs):
return self.method(*args, **kwargs)
For one I think this makes the behavior of decorators very clear, but it also makes it easy to define new decorators very concisely. For the example listed above, you could then solve it as:
class MakeBold(Decorator):
def call():
return "<b>" + self.method() + "</b>"
class MakeItalic(Decorator):
def call():
return "<i>" + self.method() + "</i>"
#MakeBold()
#MakeItalic()
def say():
return "Hello"
You could also use it to do more complex tasks, like for instance a decorator which automatically makes the function get applied recursively to all arguments in an iterator:
class ApplyRecursive(Decorator):
def __init__(self, *types):
super().__init__()
if not len(types):
types = (dict, list, tuple, set)
self._types = types
def call(self, arg):
if dict in self._types and isinstance(arg, dict):
return {key: self.call(value) for key, value in arg.items()}
if set in self._types and isinstance(arg, set):
return set(self.call(value) for value in arg)
if tuple in self._types and isinstance(arg, tuple):
return tuple(self.call(value) for value in arg)
if list in self._types and isinstance(arg, list):
return list(self.call(value) for value in arg)
return self.method(arg)
#ApplyRecursive(tuple, set, dict)
def double(arg):
return 2*arg
print(double(1))
print(double({'a': 1, 'b': 2}))
print(double({1, 2, 3}))
print(double((1, 2, 3, 4)))
print(double([1, 2, 3, 4, 5]))
Which prints:
2
{'a': 2, 'b': 4}
{2, 4, 6}
(2, 4, 6, 8)
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
Notice that this example didn't include the list type in the instantiation of the decorator, so in the final print statement the method gets applied to the list itself, not the elements of the list.

#decorator.py
def makeHtmlTag(tag, *args, **kwds):
def real_decorator(fn):
css_class = " class='{0}'".format(kwds["css_class"]) \
if "css_class" in kwds else ""
def wrapped(*args, **kwds):
return "<"+tag+css_class+">" + fn(*args, **kwds) + "</"+tag+">"
return wrapped
# return decorator dont call it
return real_decorator
#makeHtmlTag(tag="b", css_class="bold_css")
#makeHtmlTag(tag="i", css_class="italic_css")
def hello():
return "hello world"
print hello()
You can also write decorator in Class
#class.py
class makeHtmlTagClass(object):
def __init__(self, tag, css_class=""):
self._tag = tag
self._css_class = " class='{0}'".format(css_class) \
if css_class != "" else ""
def __call__(self, fn):
def wrapped(*args, **kwargs):
return "<" + self._tag + self._css_class+">" \
+ fn(*args, **kwargs) + "</" + self._tag + ">"
return wrapped
#makeHtmlTagClass(tag="b", css_class="bold_css")
#makeHtmlTagClass(tag="i", css_class="italic_css")
def hello(name):
return "Hello, {}".format(name)
print hello("Your name")

Here is a simple example of chaining decorators. Note the last line - it shows what is going on under the covers.
############################################################
#
# decorators
#
############################################################
def bold(fn):
def decorate():
# surround with bold tags before calling original function
return "<b>" + fn() + "</b>"
return decorate
def uk(fn):
def decorate():
# swap month and day
fields = fn().split('/')
date = fields[1] + "/" + fields[0] + "/" + fields[2]
return date
return decorate
import datetime
def getDate():
now = datetime.datetime.now()
return "%d/%d/%d" % (now.day, now.month, now.year)
#bold
def getBoldDate():
return getDate()
#uk
def getUkDate():
return getDate()
#bold
#uk
def getBoldUkDate():
return getDate()
print getDate()
print getBoldDate()
print getUkDate()
print getBoldUkDate()
# what is happening under the covers
print bold(uk(getDate))()
The output looks like:
17/6/2013
<b>17/6/2013</b>
6/17/2013
<b>6/17/2013</b>
<b>6/17/2013</b>

Speaking of the counter example - as given above, the counter will be shared between all functions that use the decorator:
def counter(func):
def wrapped(*args, **kws):
print 'Called #%i' % wrapped.count
wrapped.count += 1
return func(*args, **kws)
wrapped.count = 0
return wrapped
That way, your decorator can be reused for different functions (or used to decorate the same function multiple times: func_counter1 = counter(func); func_counter2 = counter(func)), and the counter variable will remain private to each.

Decorate functions with different number of arguments:
def frame_tests(fn):
def wrapper(*args):
print "\nStart: %s" %(fn.__name__)
fn(*args)
print "End: %s\n" %(fn.__name__)
return wrapper
#frame_tests
def test_fn1():
print "This is only a test!"
#frame_tests
def test_fn2(s1):
print "This is only a test! %s" %(s1)
#frame_tests
def test_fn3(s1, s2):
print "This is only a test! %s %s" %(s1, s2)
if __name__ == "__main__":
test_fn1()
test_fn2('OK!')
test_fn3('OK!', 'Just a test!')
Result:
Start: test_fn1
This is only a test!
End: test_fn1
Start: test_fn2
This is only a test! OK!
End: test_fn2
Start: test_fn3
This is only a test! OK! Just a test!
End: test_fn3

Paolo Bergantino's answer has the great advantage of only using the stdlib, and works for this simple example where there are no decorator arguments nor decorated function arguments.
However it has 3 major limitations if you want to tackle more general cases:
as already noted in several answers, you can not easily modify the code to add optional decorator arguments. For example creating a makestyle(style='bold') decorator is non-trivial.
besides, wrappers created with #functools.wraps do not preserve the signature, so if bad arguments are provided they will start executing, and might raise a different kind of error than the usual TypeError.
finally, it is quite difficult in wrappers created with #functools.wraps to access an argument based on its name. Indeed the argument can appear in *args, in **kwargs, or may not appear at all (if it is optional).
I wrote decopatch to solve the first issue, and wrote makefun.wraps to solve the other two. Note that makefun leverages the same trick than the famous decorator lib.
This is how you would create a decorator with arguments, returning truly signature-preserving wrappers:
from decopatch import function_decorator, DECORATED
from makefun import wraps
#function_decorator
def makestyle(st='b', fn=DECORATED):
open_tag = "<%s>" % st
close_tag = "</%s>" % st
#wraps(fn)
def wrapped(*args, **kwargs):
return open_tag + fn(*args, **kwargs) + close_tag
return wrapped
decopatch provides you with two other development styles that hide or show the various python concepts, depending on your preferences. The most compact style is the following:
from decopatch import function_decorator, WRAPPED, F_ARGS, F_KWARGS
#function_decorator
def makestyle(st='b', fn=WRAPPED, f_args=F_ARGS, f_kwargs=F_KWARGS):
open_tag = "<%s>" % st
close_tag = "</%s>" % st
return open_tag + fn(*f_args, **f_kwargs) + close_tag
In both cases you can check that the decorator works as expected:
#makestyle
#makestyle('i')
def hello(who):
return "hello %s" % who
assert hello('world') == '<b><i>hello world</i></b>'
Please refer to the documentation for details.

I add a case when you need to add custom parameters in decorator, pass it to final function and then work it with.
the very decorators:
def jwt_or_redirect(fn):
#wraps(fn)
def decorator(*args, **kwargs):
...
return fn(*args, **kwargs)
return decorator
def jwt_refresh(fn):
#wraps(fn)
def decorator(*args, **kwargs):
...
new_kwargs = {'refreshed_jwt': 'xxxxx-xxxxxx'}
new_kwargs.update(kwargs)
return fn(*args, **new_kwargs)
return decorator
and the final function:
#app.route('/')
#jwt_or_redirect
#jwt_refresh
def home_page(*args, **kwargs):
return kwargs['refreched_jwt']

Yet another example of nested decorators for plotting an image:
import matplotlib.pylab as plt
def remove_axis(func):
def inner(img, alpha):
plt.axis('off')
func(img, alpha)
return inner
def plot_gray(func):
def inner(img, alpha):
plt.gray()
func(img, alpha)
return inner
#remove_axis
#plot_gray
def plot_image(img, alpha):
plt.imshow(img, alpha=alpha)
plt.show()
Now, let's show a color image first without axis labels using the nested decorators:
plot_image(plt.imread('lena_color.jpg'), 0.4)
Next, let's show a gray scale image without axis labels using the nested decorators remove_axis and plot_gray (we need to cmap='gray', otherwise the default colormap is viridis, so a grayscale image is by default not displayed in black and white shades, unless explicitly specified)
plot_image(plt.imread('lena_bw.jpg'), 0.8)
The above function call reduces down to the following nested call
remove_axis(plot_gray(plot_image))(img, alpha)

With make_bold() and make_italic() below:
def make_bold(func):
def core(*args, **kwargs):
result = func(*args, **kwargs)
return "<b>" + result + "</b>"
return core
def make_italic(func):
def core(*args, **kwargs):
result = func(*args, **kwargs)
return "<i>" + result + "</i>"
return core
You can use them as decorators with say() as shown below:
#make_bold
#make_italic
def say():
return "Hello"
print(say())
Output:
<b><i>Hello</i></b>
And of course, you can directly use make_bold() and make_italic() without decorators as shown below:
def say():
return "Hello"
f1 = make_italic(say)
f2 = make_bold(f1)
result = f2()
print(result)
In short:
def say():
return "Hello"
result = make_bold(make_italic(say))()
print(result)
Output:
<b><i>Hello</i></b>

Consider the following decorator, note that we are returning the wrapper() function as an object
def make_bold(func):
def wrapper():
return '<b>'+func()+'</b>'
return wrapper
So This
#make_bold
def say():
return "Hello"
evaluates to this
x = make_bold(say)
Note that x is not the say() but the wrapper object that calls say() internally. That is how decorator works. It always returns the wrapper object which calls the actual function.
In case of chaining this
#make_italic
#make_bold
def say():
return "Hello"
gets converted to this
x = make_bold(say)
y = make_italic(x)
Below is the complete code
def make_italic(func):
def wrapper():
return '<i>'+func()+'</i>'
return wrapper
def make_bold(func):
def wrapper():
return '<b>'+func()+'</b>'
return wrapper
#make_italic
#make_bold
def say():
return "Hello"
if __name__ == '__main__':
# x = make_bold(say) When you wrap say with make_bold decorator
# y = make_italic(x) When you also add make_italic as part of chaining
# print(y())
print(say())
The above code will return
<i><b>Hello</b></i>
Hope this helps