为什么建议在Python中从Exception而不是BaseException类派生？ [英] Why is it recommended to derive from Exception instead of BaseException class in Python?
问题描述
Python 2文档说，鼓励程序员从Exception类或其子类之一而不是BaseException派生新的异常。
The Python 2 documentation says that "programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException". Without any further explanation as to why.
我很好奇为什么建议采用这种方式？仅仅是为了保留例外层次结构，因为Python开发人员已经设想到了它？
I am curious as to why it is recommended this way? Is it just to preserve the exceptions hierarchy as Python developers envisioned it?
>>> dir(BaseException) == dir(Exception)
True
推荐答案
从 BaseException 派生的异常是： GeneratorExit ， KeyboardInterrupt ， SystemExit 。
Exceptions derived from BaseException are: GeneratorExit, KeyboardInterrupt, SystemExit.
根据文档：
-
GeneratorExit：在调用生成器的close（）方法时引发。它直接从BaseException继承，而不是从StandardError继承，因为从技术上讲它不是错误。 -
KeyboardInterrupt：当用户按下中断键（通常为Control-C或Delete）时引发。在执行期间，会定期检查中断。当内置函数input（）或raw_input（）等待输入时键入的中断也会引发此异常。 该异常继承自BaseException，以免被捕获Exception的代码意外捕获，从而防止解释器退出。 -
SystemExit：该异常继承自BaseException而不是StandardError或Exception这样它就不会被捕获Exception的代码意外捕获。 这允许异常正确传播并导致解释器退出。
GeneratorExit: Raised when a generator‘s close() method is called. It directly inherits from BaseException instead of StandardError since it is technically not an error.KeyboardInterrupt: Raised when the user hits the interrupt key (normally Control-C or Delete). During execution, a check for interrupts is made regularly. Interrupts typed when a built-in function input() or raw_input() is waiting for input also raise this exception. The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.SystemExit: The exception inherits from BaseException instead of StandardError or Exception so that it is not accidentally caught by code that catches Exception. This allows the exception to properly propagate up and cause the interpreter to exit.
所以通常的原因是防止 try ...，除了例外意外阻止解释器退出（ GeneratorExit 除外）
So the usual reasons are to prevent try ... except Exception accidently prevent interpreter exit (except GeneratorExit)
更新：
异常层次结构现在更加重要，因为它具有
基本根，因此需要对现有层次结构进行更改。就目前的情况而言，如果
想要捕获表示错误
的所有异常并且并不意味着应允许解释器退出，则您必须
特别指定一个中除两个异常外的所有异常。 else子句或
分别捕获这两个异常，然后重新引发它们，并使
所有其他异常都落入一个空的except子句：
With the exception hierarchy now even more important since it has a basic root, a change to the existing hierarchy is called for. As it stands now, if one wants to catch all exceptions that signal an error and do not mean the interpreter should be allowed to exit, you must specify all but two exceptions specifically in an except clause or catch the two exceptions separately and then re-raise them and have all other exceptions fall through to a bare except clause:
except (KeyboardInterrupt, SystemExit):
raise
except:
...
这是不必要的。该PEP建议将
KeyboardInterrupt和SystemExit移至直接从
BaseException继承。
That is needlessly explicit. This PEP proposes moving KeyboardInterrupt and SystemExit to inherit directly from BaseException.
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