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# 9.2 创建装饰器时保留函数元信息

## 问题

你写了一个装饰器作用在某个函数上，但是这个函数的重要的元信息比如名字、文档字符串、注解和参数签名都丢失了。

## 解决方案

任何时候你定义装饰器的时候，都应该使用 `functools` 库中的 `@wraps` 装饰器来注解底层包装函数。例如：

```python
import time
from functools import wraps
def timethis(func):
    '''
    Decorator that reports the execution time.
    '''
    @wraps(func)
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()
        print(func.__name__, end-start)
        return result
    return wrapper
```

下面我们使用这个被包装后的函数并检查它的元信息：

```python
>>> @timethis
... def countdown(n):
...     '''
...     Counts down
...     '''
...     while n > 0:
...         n -= 1
...
>>> countdown(100000)
countdown 0.008917808532714844
>>> countdown.__name__
'countdown'
>>> countdown.__doc__
'\n\tCounts down\n\t'
>>> countdown.__annotations__
{'n': <class 'int'>}
```

## 讨论

`@wraps` 有一个重要特征是它能让你通过属性 `__wrapped__` 直接访问被包装函数。例如:

```python
>>> countdown.__wrapped__(100000)
```

`__wrapped__` 属性还能让被装饰函数正确暴露底层的参数签名信息。例如：

```python
>>> from inspect import signature
>>> print(signature(countdown))
(n:int)
```


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