Python自定义异常及子异常

Question

有什么好的方法来抛出子异常（如果这个词是对的）？ 场景： 我想在发生http或ftp异常时抛出一个自定义的连接异常（ConnectivityException）。有没有办法抛出连接异常，让这些异常能够被正确分类（也就是说，我应该能分辨出连接异常是因为http还是ftp引起的）？

Answer 1

你可以给ConnectivityException这个错误加一个叫做'source'的属性，根据具体情况把它设置为'http'或者'ftp'。当你捕捉到这个错误时，可以检查'source'属性，然后决定接下来要怎么处理。

在这里，我推荐另一种方法，就是使用继承类。

class ConnectivityException(Exception):
    pass # you can define some attributes and methods, here I just escape

class HTTPConnectivityException(ConnectivityException):
    pass

class FTPConnectivityException(ConnectivityException):
    pass

def func():
    if some_condition:
        raise HTTPConnectivityException()
    if some_other_condition:
        raise FTPConnectivityException()

def another_func():
    try:
        func()
    except HTTPConnectivityException as e:
        pass # you can do something here
    except FTPConnectivityException as e:
        pass

Answer 2

一种常见的方法是创建一个新的异常类，专门处理不同类型的连接错误，这样你可以通过继承ConnectivityException来实现：

class ConnectivityException(Exception): pass

class HTTPConnectivityException(ConnectivityException): pass

class FTPConnectivityException(ConnectivityException): pass

这样一来，当你遇到特定的错误时，就可以用raise HTTPConnectivityException或者raise FTPConnectivityException来抛出相应的错误，而不是总是用raise ConnectivityException。

你可以使用多个异常处理块，根据不同的异常类型来处理错误：

try:
    some_network_operation()
except HTTPConnectivityException as ex:
    # This will execute if the error is an HTTPConnectivityException.
except FTPConnectivityException as ex:
    # Likewise for FTPConnectivityException.
except ConnectivityException as ex:
    # The generic case; this block will execute if the ConnectivityException isn't 
    # an instance of one of the earlier specified subclasses.

需要注意的是，异常处理块是按照顺序尝试的；第一个匹配到的异常类会被使用。所以在这种情况下，你需要把ConnectivityException的处理块放在最后，否则它会捕捉到HTTPConnectivityException和FTPConnectivityException的错误。