预计阅读本页时间：-
Providing Exception Details
When an exception is raised, it may cross arbitrary file boundaries—the raise statement that triggers an exception and the try statement that catches it may be in completely different module files. It is not generally feasible to store extra details in global variables because the try statement might not know which file the globals reside in. Passing extra state information along in the exception itself allows the try statement to access it more reliably.
With classes, this is nearly automatic. As we’ve seen, when an exception is raised, Python passes the class instance object along with the exception. Code in try statements can access the raised instance by listing an extra variable after the as keyword in an except handler. This provides a natural hook for supplying data and behavior to the handler.
广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元
For example, a program that parses data files might signal a formatting error by raising an exception instance that is filled out with extra details about the error:
>>> class FormatError(Exception):
... def __init__(self, line, file):
... self.line = line
... self.file = file
...
>>> def parser():
... raise FormatError(42, file='spam.txt') # When error found
...
>>> try:
... parser()
... except FormatError as X:
... print('Error at', X.file, X.line)
...
Error at spam.txt 42
In the except clause here, the variable X is assigned a reference to the instance that was generated when the exception was raised.[78] This gives access to the attributes attached to the instance by the custom constructor. Although we could rely on the default state retention of built-in superclasses, it’s less relevant to our application:
>>> class FormatError(Exception): pass # Inherited constructor
...
>>> def parser():
... raise FormatError(42, 'spam.txt') # No keywords allowed!
...
>>> try:
... parser()
... except FormatError as X:
... print('Error at:', X.args[0], X.args[1]) # Not specific to this app
...
Error at: 42 spam.txt
