if you have ever been interviewed for a developer position, you heard this question: What is a python decorator? If you google " ", you can find various answers to this question and a lot of them are incorrect. For example from python developer interview questions here A decorator is a special kind of function that either takes a function and returns a function, or takes a class and returns a class. Well, nope. A decorator is a special kind of function It can be any callable that either takes a function and returns a function it can return anything it can also take a method and in some cases it can take anything or takes a class and returns a class. same, it can return anything Let's dive into python decorators and find out what they can do. I will not cover the basics, like decorator with parameters, I will give some strange examples to illustrate useful cases. Official definition If you look into and you will see, that decorators are just syntax sugar for this - PEP 318 -- Decorators for Functions and Methods PEP 3129 -- Class Decorators foo = decorator(foo) : def foo (self) #perform method operation # Can be replaced with this @decorator : def foo (self) #perform method operation and this - Foo = decorator(Foo) : class Foo pass # Can be replaced with this @decorator : class Foo pass Looks pretty simple and a lot of people underestimate the power of decorators. The trick is there are no restrictions for the function. decorator Most of the examples in the internet will show you something like this - print( , func.__name__, , args, , kwargs) result = func(*args, **kwargs) print( , func.__name__, , result) result wrapper a * b mul( , ) mul( , b= ) : def print_decorator (func) : def wrapper (*args, **kwargs) 'function' 'called with args - ' 'and kwargs - ' 'function' 'returns' return return @print_decorator : def mul (a, b) return 2 2 ### function mul called with args - (2, 2) and kwargs - {} ### function mul returns 4 ### 4 3 5 ### function mul called with args - (3,) and kwargs - {'b': 5} ### function mul returns 15 ### 15 ### However you can easily do this - print(func) print(func2) @type : def func () # func will be func = type(func) -> <class 'function'> return 42 ### <class 'function'> @print : def func2 () # print doesn't return anything, so func == None return 42 ### <function func2 at 0x7fd9c4788950> ### None ### Yes, i just used build-ins and as decorators. It is useless, and you will never do such nonsense, but it is possible. type print Essentially that accept class, method or function can be a decorator. anything Decorator as design pattern From book - GoF Decorator Intent Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. If you want to know more online from free sources, you can check this out In short, a decorator pattern wraps an object and dynamically adds some functionality to it. This is one of the roots of the confusion surrounding decorators in python, because they have the same name, can be used to implement the same idea, but can do a lot more. Decorator features in python Let's look at some characteristics of decorators in Python Decorator is the syntax feature, that can be applied to classes functions methods The decorator can do anything with a decorated object, for example add new behaviour (classical decorator pattern from GoF book) replace decorated object with something else change interface of decorated class or function (can be used as implementation of ) Adapter pattern cache function calls (for example ) functools.lru_cache create relation with other objects in system (e.g. route('/path/') decorator in flask) do nothing Any callable can be used as decorator, including functions, methods, functors SHOW ME THE CODE! Let's go over few examples Replace decorated object with something else self.func = func self.stats = [] : result = self.func(*args, **kwargs) Exception e: self.stats.append((args, kwargs, e)) e : self.stats.append((args, kwargs, result)) result cls(func) x/y func( , ) func(x= , y= ) func( , ) func.stats func : class FunctionTracker : def __init__ (self, func) : def __call__ (self, *args, **kwargs) try except as raise else return @classmethod : def track_function (cls, func) return @FunctionTracker.track_function : def func (x, y) return 4 2 ### 2.0 5 2 ### 2.5 3 0 ### Traceback (most recent call last): ### File "<input>", line 1, in <module> ### func(3, 0) ### File "<input>", line 11, in __call__ ### raise e ### File "<input>", line 8, in __call__ ### result = self.func(*args, **kwargs) ### File "<input>", line 3, in func ### return x/y ### ZeroDivisionError: division by zero ### [((4, 2), {}, 2.0), ((), {'x': 5, 'y': 2}, 2.5), ((3, 0), {}, ZeroDivisionError('division by zero',))] ### <__main__.FunctionTracker object at 0x7fd35978a668> Be careful with this example, as accumulating data in this way can lead to memory leaks. Notice how the original was replaced by an instance of , which can be used in the same way as the original function, thanks to the method. func FunctionTracker __call__ Create relation with other objects in system json base64 urllib.parse BASE64_ENCODED_JSON = URLENCODED = self._registry = {} self._registry[data_format] = encoder_function encoder_function _register_encoder self._registry[data_format](data) encoder = DataEncoder() base64.b64encode( json.dumps(data).encode( ) ) encoder.register_format(URLENCODED)(urllib.parse.urlencode) encoder._registry data = { : , : } encoder.encode(BASE64_ENCODED_JSON, data) encoder.encode(URLENCODED, data) import import import 'BASE64_ENCODED_JSON' 'URLENCODED' : class DataEncoder : def __init__ (self) # Mapping for algorithm_name and implementation : def register_format (self, data_format) : def _register_encoder (encoder_function) return return : def encode (self, data_format, data) return @encoder.register_format(BASE64_ENCODED_JSON) : def base64_encoded_json (data) return 'utf-8' # Notice that `encoder.register_format` can be used as regular function ### <function urlencode at 0x7fe9422a86a8> ### {'URLENCODED': <function urlencode at 0x7fe9422a86a8>, 'BASE64_ENCODED_JSON': <function base64_encoded_json at 0x7fe9406e0620>} 'greetings' 'Hello there' 'answer' 'General Kenobi' ### b'eyJncmVldGluZ3MiOiAiSGVsbG8gdGhlcmUiLCAiYW5zd2VyIjogIkdlbmVyYWwgS2Vub2JpIn0=' ### 'greetings=Hello+there&answer=General+Kenobi' ### In this example, the function and are connected using decorator syntax as implementations for the class. base64_encoded_json urllib.parse.urlencode DataEncoder Flask uses a similar approach for registering functions as views via decorator; Django has for registering admin classes. route('/path/') @admin.register It can also be used as an alternative to inheritance, in the case of I can create a subclass of with new encoding implementation instead, which isn't a best solution. DataEncoder DataEncoder Silly example Several decorators can be applied to one function. That means, that this @dec2 @dec1 : def func () pass equals this func = dec2(dec1(func)) : def func () pass Which means, that can accept , that can return. dec2 ANYTHING dec1 There will be weird looking code here. It is here only because I can. I would never recommend anyone use this. Disclaimer: {func.__name__: func} operations = {} a * b a / b print(mul) print(operations) operations[ ]( , ) : def dict_from_func (func) return @operations.update @dict_from_func : def mul (a, b) return @operations.update @dict_from_func : def add (a, b) return ### None ### {'mul': <function mul at 0x7fdaf17bbae8>, 'add': <function add at 0x7fdaf16a2510>} 'mul' 2 5 ### 10 Here I use the method as a decorator, even if it is not intended for this. This is possible because returns a dict, and takes a dict as an argument. dict.update dict_from_func dict.update As a result, I have all the decorated functions compiled in the operations dictionary. As a side effect - all functions are replaced with , because does not return any value. None dict.update Essentially this - a / b @operations.update @dict_from_func : def add (a, b) return equals this a / b add = operations.update(dict_from_func(add)) : def add (a, b) return You can try to win a bet with your colleagues using this example. In conclusion Decorators is amazing Python feature . You can use them for a variety of purposes. It's not just a “function or class that takes a function or class and returns a function or class”.
