9.8 异常也是类 Exceptions Are Classes Too

User-defined exceptions are identified by classes as well. Using this mechanism it is possible to create extensible hierarchies of exceptions.

用户自定义异常也可以是类。利用这个机制可以创建可扩展的异常体系。

raise Class, instance

raise instance

In the first form, instance must be an instance of Class or of a class derived from it. The second form is a shorthand for:

第一种形式中，instance 必须是 Class 或其派生类的一个实例。第二种形式是以下形式的简写：

raise instance.__class__, instance

A class in an except clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around -- an except clause listing a derived class is not compatible with a base class). For example, the following code will print B, C, D in that order:

发生的异常其类型如果是异常子句中列出的类，或者是其派生类，那么它们就是相符的（反过来说－－发生的异常其类型如果是异常子句中列出的类的基类，它们就不相符）。例如，以下代码会按顺序打印B，C，D：

class B:
    pass
class C(B):
    pass
class D(C):
    pass

for c in [B, C, D]:
    try:
        raise c()
    except D:
        print "D"
    except C:
        print "C"
    except B:
        print "B"

Note that if the except clauses were reversed (with "except B" first), it would have printed B, B, B -- the first matching except clause is triggered.

要注意的是如果异常子句的顺序颠倒过来（ "execpt B"在最前），它就会打印B，B，B－－第一个匹配的异常被触发。

When an error message is printed for an unhandled exception which is a class, the class name is printed, then a colon and a space, and finally the instance converted to a string using the built-in function str().

打印一个异常类的错误信息时，先打印类名，然后是一个空格、一个冒号，然后是用内置函数 str() 将类转换得到的完整字符串。