python装饰器和包装器 [英] python decorators and wrappers

问题描述

我很难抓住装饰器，最初我以为装饰器是语法糖，可以执行以下操作：

I am having a hard time grasping decorators, I initially thought decorators were syntactic sugar to do an operation such as:

def decorator(x):
    return x*2

@decorator
def plusone(x):
    return x+1

print plusone(5)
# would print twelve, because: decorator(plusone(5)) as I have seen 
# in some tutorials

但是我注意到必须先在装饰器中创建包装函数。

but I've noticed that a wrapper function needs to be created in the decorator first.

为什么需要返回一个函数，而不是整数？

Why do we need to return a function, not an integer?

推荐答案

（Python）修饰符是函数修改函数的语法糖，

A (Python) decorator is syntactic sugar for a function modifying a function, causing the latter function to act like a modified version of itself.

让我们从一个简单的例子开始（不是很好）。假设我们写了

Let's start with a simple (and not so good) example. Say we write

def double_me(f):
    return lambda x: 2 * f(x)

它接受一个带有一个参数的函数，并返回一个带有一个参数的函数，并返回原始函数返回值的两倍。然后我们可以这样使用它：

which takes a function taking one parameter, and returns a function taking one parameter and returning double what the original function would return. Then we can use it like this:

def double_the_add_one(x):
    return x + 1
double_the_add_one = double_me(double_the_add_one)

>>> double_the_add_one(1)
4

请注意，我们首先定义了 double_the_add_one ，然后修改 double_the_add_one 并将其绑定回 double_the_add_one 。因此，现在它只是其本身的修改版本-它会执行其最初的操作，然后将结果加倍。装饰者只是简单地使它更加简洁：

Note that we first defined double_the_add_one, then modified double_the_add_one and bound it back to double_the_add_one. So now it is just a modified version of itself - it does what it would do originally, then just doubles the result. Decorators simply make this a bit more succinct:

@double_me 
def double_the_add_one(x):
    return x + 1

>>> double_the_add_one(1)
4

---

注意上面的装饰器不是那么好：

---

Note that the above decorator wasn't that great:

1. 它假定有一个位置参数。

1. It assumed there was a single positional argument.

假定没有关键字参数。

如果修饰的函数具有文档字符串，则该文件将被破坏。

If the decorated function had a docstring, it was clobbered.

您可能应该使用 functools.wraps 实际使用那里的文档中的示例编写装饰器。

You should probably use functools.wraps to actually write decorators using the example in the docs there.

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