使用类与函数的Python装饰器最佳实践 [英] Python decorator best practice, using a class vs a function

问题描述

据我了解，有两种方法可以使用Python装饰器，或者使用类的 __ call __ 或将函数定义并调用为装饰器。这些方法的优点/缺点是什么？有一种首选方法吗？

As I've understood it there are two ways to do a Python decorator, to either use the __call__ of a class or to define and call a function as the decorator. What's the advantages/disadvantages of these methods? Is there one preferred method?

示例1

class dec1(object):
    def __init__(self, f):
        self.f = f
    def __call__(self):
        print "Decorating", self.f.__name__
        self.f()

@dec1
def func1():
    print "inside func1()"

func1()

# Decorating func1
# inside func1()

示例2

def dec2(f):
    def new_f():
        print "Decorating", f.__name__
        f()
    return new_f

@dec2
def func2():
    print "inside func2()"

func2()

# Decorating func2
# inside func2()

推荐答案

说是否有优势是相当主观的。

It is rather subjective to say whether there are "advantages" to each method.

但是，深入了解幕后的事物会使
自然成为一个人在每种情况下选择最佳选择。

However, a good understanding of what goes under the hood would make it natural for one to pick the best choice for each occasion.

装饰器（谈论函数装饰器），只是一个将函数作为其输入参数的可调用对象。 Python有其相当有趣的设计，它允许
除了函数之外还可以创建其他类型的可调用对象-并且可以使用
来偶尔创建可维护性或更短的代码。

A decorator (talking about function decorators), is simply a callable object that takes a function as its input parameter. Python has its rather interesting design that allows one to create other kinds of callable objects, besides functions - and one can put that to use to create more maintainable or shorter code on occasion.

装饰器是在Python 2.3中作为语法快捷方式添加回来的。对于

Decorators were added back in Python 2.3 as a "syntactic shortcut" for

def a(x):
   ...

a = my_decorator(a)

除此之外，我们通常将装饰器称为可调用对象。宁愿是装饰工厂 -当我们使用这种类型时：

Besides that, we usually call decorators some "callables" that would rather be "decorator factories" - when we use this kind:

@my_decorator(param1, param2)
def my_func(...):
   ...

将呼叫 my_decorator使用param1和param2-然后返回一个对象，该对象将再次被调用，这次具有 my_func属性。作为参数。因此，在这种情况下，从技术上讲，装饰器是指是 my_decorator所返回的内容，使其成为
decorator factory。

the call is made to "my_decorator" with param1 and param2 - it then returns an object that will be called again, this time having "my_func" as a parameter. So, in this case, technically the "decorator" is whatever is returned by the "my_decorator", making it a "decorator factory".

现在，装饰器或 decorator factory如上所述通常必须保持一些内部状态。在第一种情况下，它唯一保留的是对原始函数的引用（在示例中称为 f 的变量）。一个装饰工厂。可能希望注册额外的状态变量（在上面的示例中为 param1和 param2）。

Now, either decorators or "decorator factories" as described usually have to keep some internal state. In the first case, the only thing it does keep is a reference to the original function (the variable called f in your examples). A "decorator factory" may want to register extra state variables ("param1" and "param2" in the example above).

在以函数形式编写的装饰器的情况下，此额外状态保留在变量中在封闭功能内，并且以非本地访问。实际包装函数的变量。如果编写适当的类，则可以将它们作为实例变量保存在装饰器函数（将被视为可调用对象，而不是函数）中，并且对它们的访问更加明确和可读。

This extra state, in the case of decorators written as functions is kept in variables within the enclosing functions, and accessed as "nonlocal" variables by the actual wrapper function. If one writes a proper class, they can be kept as instance variables in the decorator function (which will be seen as a "callable object", not a "function") - and access to them is more explicit and more readable.

因此，在大多数情况下，是否选择一种方法还是可读性问题：
是简短，简单的装饰器，功能性方法通常比一类写成一类，而有时写得更精细一类，特别是一类装饰工厂。将充分利用扁平优于嵌套的优势。

So, for most cases it is a matter of readability whether you will prefer one approach or the other: for short, simple decorators, the functional approach is often more readable than one written as a class - while sometimes a more elaborate one - especially one "decorator factory" will take full advantage of the "flat is better than nested" advice fore Python coding.

考虑：

def my_dec_factory(param1, param2):
   ...
   ...
   def real_decorator(func):
       ...
       def wraper_func(*args, **kwargs):
           ...
           #use param1
           result = func(*args, **kwargs)
           #use param2
           return result
       return wraper_func
   return real_decorator

针对此混合解决方案：

class MyDecorator(object):
    """Decorator example mixing class and function definitions."""
    def __init__(self, func, param1, param2):
        self.func = func
        self.param1, self.param2 = param1, param2

    def __call__(self, *args, **kwargs):
        ...
        #use self.param1
        result = self.func(*args, **kwargs)
        #use self.param2
        return result

def my_dec_factory(param1, param2):
    def decorator(func):
         return MyDecorator(func, param1, param2)
    return decorator

更新：缺少纯类；装饰形式

update: Missing "pure class" forms of decorators

现在，请注意混合该方法采用了两全其美的方法。尝试保留最短和更易读的代码。完整的装饰工厂用类专门定义的类要么需要两个类，要么需要一个模式。属性以了解是否调用了该方法来注册修饰的函数或实际调用最终函数：

Now, note the "hybrid" method takes the "best of both Worlds" trying to keep the shortest and more readable code. A full "decorator factory" defined exclusively with classes would either need two classes, or a "mode" attribute to know if it was called to register the decorated function or to actually call the final function:

class MyDecorator(object):
   """Decorator example defined entirely as class."""
   def __init__(self, p1, p2):
        self.p1 = p1
        ...
        self.mode = "decorating"

   def __call__(self, *args, **kw):
        if self.mode == "decorating":
             self.func = args[0]
             self.mode = "calling"
             return self
         # code to run prior to function call
         result = self.func(*args, **kw)
         # code to run after function call
         return result

@MyDecorator(p1, ...)
def myfunc():
    ...

最后是纯净的白领用两个类定义的装饰器-也许可以使事情更加分离，但是将冗余性提高到一定程度，这不能说它更易于维护：

And finally a pure, "white colar" decorator defined with two classes - maybe keeping things more separated, but increasing the redundancy to a point one can't say it is more maintainable:

class Stage2Decorator(object):
    def __init__(self, func, p1, p2, ...):
         self.func = func
         self.p1 = p1
         ...
    def __call__(self, *args, **kw):
         # code to run prior to function call
         ...
         result = self.func(*args, **kw)
         # code to run after function call
         ...
         return result

class Stage1Decorator(object):
   """Decorator example defined as two classes.
   
   No "hacks" on the object model, most bureacratic.
   """
   def __init__(self, p1, p2):
        self.p1 = p1
        ...
        self.mode = "decorating"

   def __call__(self, func):
       return Stage2Decorator(func, self.p1, self.p2, ...)


@Stage1Decorator(p1, p2, ...)
def myfunc():
    ...

2018年更新

几年前我在上面写下了文字。最近，由于创建了更扁平的代码，我想出了一个我喜欢的模式。

2018 update

I wrote the text above a couple years ago. I came up recently with a pattern I prefer due to creating code that is "flatter".

基本思想是使用一个函数，但返回一个 partial 本身的对象（如果在用作修饰符之前用参数调用）：

The basic idea is to use a function, but return a partial object of itself if it is called with parameters before being used as a decorator:

from functools import wraps, partial

def decorator(func=None, parameter1=None, parameter2=None, ...):

   if not func:
        # The only drawback is that for functions there is no thing
        # like "self" - we have to rely on the decorator 
        # function name on the module namespace
        return partial(decorator, parameter1=parameter1, parameter2=parameter2)
   @wraps(func)
   def wrapper(*args, **kwargs):
        # Decorator code-  parameter1, etc... can be used 
        # freely here
        return func(*args, **kwargs)
   return wrapper

就是这样-使用此模式编写的装饰器可以立即装饰
a函数，而无需被称为首先：

And that is it - decorators written using this pattern can decorate a function right away without being "called" first:

@decorator
def my_func():
    pass

或使用以下参数自定义：

Or customized with parameters:

@decorator(parameter1="example.com", ...):
def my_func():
    pass
        
        

2019 -使用Python 3.8和仅位置参数，最后一个模式会变得更好，因为 func 参数只能声明为位置，并要求命名参数；

2019 - With Python 3.8 and positional only parameters this last pattern will become even better, as the func argument can be declared as positional only, and require the parameters to be named;

def decorator(func=None, *, parameter1=None, parameter2=None, ...):

这篇关于使用类与函数的Python装饰器最佳实践的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！