9.7.1 Exceptions Can Be Classes

User-defined exceptions are no longer limited to being string objects -- they can be identified by classes as well. Using this mechanism it is possible to create extensible hierarchies of exceptions.

There are two new valid (semantic) forms for the raise statement:

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

raise instance.__class__, instance

An except clause may list classes as well as string objects. 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:

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.

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().