访问Python中装饰器参数的默认值。

huangapple
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英文:

Accessing default value of decorator argument in Python

问题

Here's the translated code without the code comments:

  1. 我正在尝试编写一个自定义的Python装饰器，它将装饰的函数包装在一个`try ... except`块中，并添加一个包含附加上下文的消息，以便更容易调试。
  3. 基于不同的资源（例如[这里](https://www.geeksforgeeks.org/decorators-with-parameters-in-python/)和[这里](https://www.freecodecamp.org/news/python-decorators-explained-with-examples/)），我到目前为止构建了以下内容：
  5. ```python
  6. def _exception_handler(msg):
  7.     """Custom decorator to return a more informative error message"""
  8.     def decorator(func):
  9.         def wrapper(*args, **kwargs):
  10.             try:
  11.                 # this is the actual decorated function
  12.                 func(*args, **kwargs)
  13.             except Exception as e:
  14.                 # we catch the exception and raise a new one with the custom
  15.                 # message and the original exception as cause
  16.                 raise Exception(f"{msg}: {e}") from e
  17.         return wrapper
  18.     return decorator

这按预期工作 - 如果我运行：

  1. @_exception_handler("Foo")
  2. def test():
  3.     raise ValueError("Bar")
  5. test()

这会返回：

  1. Exception: Foo: Bar

现在，我并不总是想传递自定义的msg，因为有时这有点多余。因此，我将msg=""设置为默认值，并且我想要检查msg==""，如果是这种情况，我只想在装饰器内重新创建msg，类似于msg = f"An error occurred in {func.__name__}"。

然后，我想在没有任何msg参数的情况下使用装饰器。我不关心空括号，对我来说使用@_exception_handler()是完全可以的。

但是某种方式似乎不起作用：

  1. def _exception_handler(msg=""):
  2.     """Custom decorator to return a more informative error message"""
  3.     def decorator(func):
  4.         def wrapper(*args, **kwargs):
  5.             try:
  6.                 # this is the actual decorated function
  7.                 func(*args, **kwargs)
  8.             except Exception as e:
  9.                 if msg=="":
  10.                     # if no custom message is passed, we just use the function name
  11.                     msg = f"An error occurred in {func.__name__}"
  12.                 # we catch the exception and raise a new one with the message
  13.                 # and the original exception as cause
  14.                 raise Exception(f"{msg}: {e}") from e
  15.         return wrapper
  16.     return decorator

如果我运行：

  1. @_exception_handler()
  2. def test():
  3.     raise ValueError("Bar")
  5. test()

我会得到：

  1. UnboundLocalError: local variable 'msg' referenced before assignment

如果我将global message放在def decorator(func):行的下面，我会得到相同的错误。如果我将其放在def wrapper(*args, **kwargs):的下面，我会得到：

  1. NameError: name 'msg' is not defined

有没有办法让这个工作？如果可能的话，我想避免使用wrapt等第三方模块。当然，使用标准库中的functools的wraps也是可以的（尽管到目前为止我还没有成功）。

英文:

I am trying to write a custom Python decorator which wraps the decorated function in a try ... except block and adds a message with additional context to make debugging easier.

Based on different resources (see here and here for example) I so far built the following:

  1. def _exception_handler(msg):
  2.     """Custom decorator to return a more informative error message"""
  3.     def decorator(func):
  4.         def wrapper(*args, **kwargs):
  5.             try:
  6.                 # this is the actual decorated function
  7.                 func(*args, **kwargs)
  8.             except Exception as e:
  9.                 # we catch the exception and raise a new one with the custom
  10.                 # message and the original exception as cause
  11.                 raise Exception(f"{msg}: {e}") from e
  12.         return wrapper
  13.     return decorator

This works as expected - if I run:

  1. @_exception_handler("Foo")
  2. def test():
  3.     raise ValueError("Bar")
  5. test()

This returns:

  1. Exception: Foo: Bar

Now, I do not always want to pass a custom msg because that's sometimes a bit redundant. So I set a default value of msg="" and I want to check if msg=="" and if that is the case I would just like to re-create the msg inside the decorator based on the function name, something like msg = f"An error occurred in {func.__name__}".

I would then like to use the decorator without any msg argument. I do not care about the empty parantheses, using @_exception_handler() is perfectly fine for me.

But somehow this does not seem to work:

  1. def _exception_handler(msg=""):
  2.     """Custom decorator to return a more informative error message"""
  3.     def decorator(func):
  4.         def wrapper(*args, **kwargs):
  5.             try:
  6.                 # this is the actual decorated function
  7.                 func(*args, **kwargs)
  8.             except Exception as e:
  9.                 if msg=="":
  10.                     # if no custom message is passed, we just use the function name
  11.                     msg = f"An error occurred in {func.__name__}"
  12.                 # we catch the exception and raise a new one with the message
  13.                 # and the original exception as cause
  14.                 raise Exception(f"{msg}: {e}") from e
  15.         return wrapper
  16.     return decorator

If I run:

  1. @_exception_handler()
  2. def test():
  3.     raise ValueError("Bar")
  5. test()

I get

  1. UnboundLocalError: local variable 'msg' referenced before assignment

If I put global message right below the def decorator(func): line, I get the same errors. If I put it below the def wrapper(*args, **kwargs):, I instead get:

  1. NameError: name 'msg' is not defined

Any ideas how I can get this to work? If possible, I would like to avoid any third-party modules such as wrapt. Using wraps from functools from the standard library is fine of course (although I did not have any luck with that so far either).

答案1

得分: 5

添加这段代码 nonlocal msg。

  1. def _exception_handler(msg=""):
  2.     """自定义装饰器以返回更详细的错误消息"""
  3.     def decorator(func):
  4.         def wrapper(*args, **kwargs):
  5.             nonlocal msg
  6.             try:
  7.                 # 这是实际被装饰的函数
  8.                 func(*args, **kwargs)
  9.             except Exception as e:
  10.                 if msg == "":
  11.                     # 如果没有传递自定义消息，我们只使用函数名称
  12.                     msg = f"在 {func.__name__} 中发生了错误"
  13.                 # 我们捕获异常并引发一个带有消息和原始异常作为原因的新异常
  14.                 raise Exception(f"{msg}: {e}") from e
  15.         return wrapper
  16.     return decorator
英文:

add this code nonlocal msg.

  1. def _exception_handler(msg=""):
  2.     """Custom decorator to return a more informative error message"""
  3.     def decorator(func):
  4.         def wrapper(*args, **kwargs):
  5.             nonlocal msg
  6.             try:
  7.                 # this is the actual decorated function
  8.                 func(*args, **kwargs)
  9.             except Exception as e:
  10.                 if msg=="":
  11.                     # if no custom message is passed, we just use the function name
  12.                     msg = f"An error occurred in {func.__name__}"
  13.                 # we catch the exception and raise a new one with the message
  14.                 # and the original exception as cause
  15.                 raise Exception(f"{msg}: {e}") from e
  16.         return wrapper
  17.     return decorator

huangapple
  • 本文由 发表于 2023年5月25日 23:35:28
  • 转载请务必保留本文链接：https://go.coder-hub.com/76334025.html
  • python
  • python-decorators
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