Functions Can Signal Conditions with raise

User-defined exceptions can also signal nonerror conditions. For instance, a search routine can be coded to raise an exception when a match is found instead of returning a status flag for the caller to interpret. In the following, the try/except/else exception handler does the work of an if/else return-value tester:

class Found(Exception): pass

def searcher():
    if ...success...:
        raise Found()
    else:
        return

try:
    searcher()
except Found:                    # Exception if item was found
    ...success...
else:                            # else returned: not found
    ...failure...

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More generally, such a coding structure may also be useful for any function that cannot return a sentinel value to designate success or failure. For instance, if all objects are potentially valid return values, it’s impossible for any return value to signal unusual conditions. Exceptions provide a way to signal results without a return value:

class Failure(Exception): pass

def searcher():
    if ...success...:
        return ...founditem...
    else:
        raise Failure()

try:
    item = searcher()
except Failure:
    ...report...
else:
    ...use item here...

Because Python is dynamically typed and polymorphic to the core, exceptions, rather than sentinel return values, are the generally preferred way to signal such conditions.