try语句的可选else子句的预期用途是什么?


当前回答

一种用法:测试一些应该引发异常的代码。

try:
    this_should_raise_TypeError()
except TypeError:
    pass
except:
    assert False, "Raised the wrong exception type"
else:
    assert False, "Didn't raise any exception"

(在实践中,这段代码应该被抽象为更通用的测试。)

其他回答

PEP 380中有一个很好的try-else示例。基本上,它归结为在算法的不同部分做不同的异常处理。

大概是这样的:

try:
    do_init_stuff()
except:
    handle_init_suff_execption()
else:
    try:
        do_middle_stuff()
    except:
        handle_middle_stuff_exception()

这允许您在异常发生的地方编写异常处理代码。

如果执行落在try的底部(如果没有异常),else块中的语句将被执行。说实话,我从来没有找到过需要。

然而,处理异常注意到:

使用else子句更好 而不是向try中添加额外的代码 因为它避免意外 捕获非异常 由被保护的代码引发 尝试…除了声明。

因此,如果你有一个方法,例如,抛出一个IOError,你想捕捉它引发的异常,但如果第一个操作成功,你还想做其他事情,而你不想从那个操作中捕获IOError,你可能会写这样的东西:

try:
    operation_that_can_throw_ioerror()
except IOError:
    handle_the_exception_somehow()
else:
    # we don't want to catch the IOError if it's raised
    another_operation_that_can_throw_ioerror()
finally:
    something_we_always_need_to_do()

如果您只是在operation_that_can_throw_ioerror()后面加上另一个other_operation_that_can_throw_ioerror(), except将捕获第二次调用的错误。如果你把它放在整个try块之后,它会一直运行,直到finally之后。else可以让你确定

第二次操作只有在没有例外的情况下才会进行, 它在最后一个块之前运行,并且 它引发的任何ioerror都不会在这里被捕获

try:
    statements # statements that can raise exceptions
except:
    statements # statements that will be executed to handle exceptions
else:
    statements # statements that will be executed if there is no exception

例子:

try:
    age=int(input('Enter your age: '))
except:
    print ('You have entered an invalid value.')
else:
    if age <= 21:
        print('You are not allowed to enter, you are too young.')
    else:
        print('Welcome, you are old enough.')

输出:

>>> 
Enter your age: a
You have entered an invalid value.
>>> RESTART
>>> 
Enter your age: 25
Welcome, you are old enough.
>>> RESTART
>>> 
Enter your age: 13
You are not allowed to enter, you are too young.
>>> 

复制自:https://geek-university.com/python/the-try-except-else-statements/

使用else样式和可读性是一个重要原因。将可能导致异常的代码放在处理异常的代码附近通常是个好主意。例如,比较这些:

try:
    from EasyDialogs import AskPassword
    # 20 other lines
    getpass = AskPassword
except ImportError:
    getpass = default_getpass

and

try:
    from EasyDialogs import AskPassword
except ImportError:
    getpass = default_getpass
else:
    # 20 other lines
    getpass = AskPassword

当异常不能提前返回或重新抛出异常时,第二个方法很好。如果可能的话,我会这样写:

try:
    from EasyDialogs import AskPassword
except ImportError:
    getpass = default_getpass
    return False  # or throw Exception('something more descriptive')

# 20 other lines
getpass = AskPassword

注意:答案从最近发布的副本复制在这里,因此所有这些“AskPassword”的东西。

我将添加另一个用例,在处理DB会话时看起来很直接:

    # getting a DB connection 
    conn = db.engine.connect()

    # and binding to a DB session
    session = db.get_session(bind=conn)

    try:
        # we build the query to DB
        q = session.query(MyTable).filter(MyTable.col1 == 'query_val')

        # i.e retrieve one row
        data_set = q.one_or_none()

        # return results
        return [{'col1': data_set.col1, 'col2': data_set.col2, ...}]

    except:
        # here we make sure to rollback the transaction, 
        # handy when we update stuff into DB
        session.rollback()
        raise

    else:
        # when no errors then we can commit DB changes
        session.commit()

    finally:
        # and finally we can close the session
        session.close()