python - 如何防止多次捕获异常

Question

有没有办法防止在堆栈上到达程序顶层时多次捕获异常？

这是一个非常简化的代码示例，说明了这种现象:

def try_except_block(smthg):
   try:
     smthg.run()
   except Exception as e:
     print("WRONG")
     raise e

def re_call():
   f2 = Foo(True) # will throw an exception
   try_except_block(f2)

class Foo:
  def __init__(self, t):
    self.throw = t
  def run(self):
    if self.throw:
      raise KeyError
    else:
      re_call()

if __name__ == '__main__':
  f = Foo(False) # won't throw an exception
  try_except_block(f)

输出:

WRONG
WRONG
Traceback (most recent call last):
  File "exception_loosing_args.py", line 26, in <module>
    try_except_block(f)
  File "exception_loosing_args.py", line 9, in try_except_block
    raise e
  File "exception_loosing_args.py", line 6, in try_except_block
    smthg.run()
  File "exception_loosing_args.py", line 22, in run
    re_call()
  File "exception_loosing_args.py", line 13, in re_call
    try_except_block(f2)
  File "exception_loosing_args.py", line 9, in try_except_block
    raise e
  File "exception_loosing_args.py", line 6, in try_except_block
    smthg.run()
  File "exception_loosing_args.py", line 20, in run
    raise KeyError
KeyError

我只想打印一次“错误”。

在我的软件中，try_except_block函数被包装到一个对象中，我有想法设置该对象的一个​​属性，该属性可以将异常语句标记为“已访问”或未访问。我不喜欢这个想法，因为这种做法可能会带来副作用。还有其他的吗？

我的想法示例(类属性由全局变量 flag 模拟):

flag = False

def try_except_block(smthg):
   global flag
   try:
     smthg.run()
   except Exception as e:
     if not flag:
       print("WRONG")
       flag = True
     raise e

def re_call():
   f2 = Foo(True) # will throw an exception
   try_except_block(f2)

class Foo:
  def __init__(self, t):
    self.throw = t
  def run(self):
    if self.throw:
      raise KeyError
    else:
      re_call()

if __name__ == '__main__':
  f = Foo(False) # won't throw an exception
  try_except_block(f)

输出:

WRONG
Traceback (most recent call last):
  File "exception_loosing_args.py", line 28, in <module>
    try_except_block(f)
  File "exception_loosing_args.py", line 11, in try_except_block
    raise e
  File "exception_loosing_args.py", line 6, in try_except_block
    smthg.run()
  File "exception_loosing_args.py", line 24, in run
    re_call()
  File "exception_loosing_args.py", line 15, in re_call
    try_except_block(f2)
  File "exception_loosing_args.py", line 11, in try_except_block
    raise e
  File "exception_loosing_args.py", line 6, in try_except_block
    smthg.run()
  File "exception_loosing_args.py", line 22, in run
    raise KeyError
KeyError

Answer 1

我不完全理解为什么你的代码会做它所做的事情，但是你可以做的可能适合的事情是将异常对象标记为已经看到的，而不是设置全局标志。就像这样:

def try_except_block(smthg):
    try:
        smthg.run()
    except Exception as e:
        if not hasattr(e, "teb_already_seen"):
            setattr(e, "teb_already_seen", True)
            print("WRONG")
        raise e

这样，try_except_block 完成的额外处理将只发生一次每个异常对象，这可能就是您想要的。