在 Python 中的异常处理程序中捕获异常的更简洁的方法

Question

在 Python 程序中，通常使用 try-except 块捕获异常：

try:
    # Do stuff
except ValueError:
    # Handle exception

我知道在异常处理程序中捕获异常的最佳方法是嵌套的 try-except 块。但是，对于许多嵌套的 try-catch 块，这可能会有点混乱：

try:
    # Some assignment that throws an exception if the object is not in the list
    try:
        # Some assignment function that throws an exception if the the object is not already in the database
        # Error Handling
    except ValueError:
        try:
            # Some function that throws an exception if the object does not have an undesired property
            # Error Handling
        except AttributeError:
            try:
                # Some function that throws an exception if an error happens
            except Exception:
                # Exception handling
except ValueError:
    # Exception handling

有没有更简洁的方法来做到这一点？比如：

try:
   # Some assignment that throws an exception if the object is not in the list
   try:
       # Some assignment function that throws an exception if the object is not already in the database
   except ValueError:
       # Some function that throws an exception if the object does not have an undesired property
   exceptexcept AttributeError:
       # Some function that throws an exception if an error happens
   exceptexcept Exception:
       # Exception handling 
except ValueError:
   # Exception handling

Answer 1

这听起来像是一个循环，您想继续尝试直到成功或用尽选项。所以你可以这样实现它，例如，像这样

# Each pair contains a function to call and an exception that can be caught.
# If that exception is raised, the next function will be tried.
action_list = [
    (get_from_list, ValueError),  # throws ValueError if item can't be retrieved
    (get_from_database, ValueError),  # throws ValueError if item can't be retrieved
    (get_from_object, AttributeError),  # throws AttributeError if item lacks desired property
]

result = None
for action, ex in action_list:
    try:
        result = action(key)
        break
    except ex:
        continue

您可以通过让所有辅助函数引发自定义异常（如“NotFound”）来稍微整理一下，然后将其用作检查下一个级别的信号，如下所示：

# actions to try; all raise NotFound if unsuccessful
action_list = [
    get_from_list, get_from_database, get_from_object
]

result = None
for action in action_list:
    try:
        result = action(key)
        break
    except NotFound:
        continue

或者您可以将所有步骤放在一个函数中，该函数一旦成功就返回。这样你的任务可以在常规代码中完成，而不是使用辅助函数：

def get_value(key):

    try:
        return some_list[int(key)]
    except ValueError:
        pass

    try:
        return get_from_database(key)
    except ValueError:
        pass

    try:
        return getattr(some_object, key)
    except AttributeError:
        pass

    return None

如果您不想要其他功能，您可以滥用for 循环：

result = None
for _ in range(1):

    try:
        result = some_list[int(key)]
        break
    except ValueError:
        pass

    try:
        result = get_from_database(key)
        break
    except ValueError:
        pass

    try:
        result = getattr(some_object, key)
        break
    except AttributeError:
        pass

或者您可以使用单个外部 try/except 和自定义 Found 异常作为“structured goto”：

result = None
try:
    try:
        result = some_list[int(key)]
        raise Found
    except ValueError:
        pass
    try:
        result = get_from_database(key)
        raise Found
    except ValueError:
        pass
    try:
        result = getattr(some_object, key)
        raise Found
    except AttributeError:
        pass
except Found:
    pass  # all good

或者有这个久经考验的构造：

result = None
if result is None:
    try:
        result = some_list[int(key)]
    except ValueError:
        pass
if result is None:
    try:
        result = get_from_database(key)
    except ValueError:
        pass
if result is None:
    try:
        result = getattr(some_object, key)
    except AttributeError:
        pass

Answer 2

是的。您可以在 python 中使用异常层次结构。 应注意例外的顺序。捕获错误的 except 将被执行，其余的都将被忽略。

做，

inspect.getclasstree(inspect.getmro(Exception))

 BaseException
... Exception
...... StandardError
......... TypeError
......... ImportError
............ ZipImportError
......... EnvironmentError
............ IOError
............... ItimerError
............ OSError
......... EOFError
......... RuntimeError
............ NotImplementedError
......... NameError
............ UnboundLocalError
......... AttributeError
......... SyntaxError
............ IndentationError
............... TabError
......... LookupError
............ IndexError
............ KeyError
............ CodecRegistryError
......... ValueError
............ UnicodeError
............... UnicodeEncodeError
............... UnicodeDecodeError
............... UnicodeTranslateError
......... AssertionError
......... ArithmeticError
............ FloatingPointError
............ OverflowError
............ ZeroDivisionError
......... SystemError
............ CodecRegistryError
......... ReferenceError
......... MemoryError
......... BufferError