else:块令人困惑并且(几乎)毫无用处。它也是for和while语句的一部分。

实际上，即使在if语句中，else:也可能被滥用，造成非常难以发现的错误。

考虑这一点。

   if a < 10:
       # condition stated explicitly
   elif a > 10 and b < 10:
       # condition confusing but at least explicit
   else:
       # Exactly what is true here?
       # Can be hard to reason out what condition is true

三思而后行。这通常是个问题。除非在if语句中，否则避免使用它，即使在if语句中，也要考虑记录else-条件，使其显式。

假设您的编程逻辑取决于字典中是否有具有给定键的条目。你可以使用if…来测试dict.get(key)的结果。其他的……或者你可以这样做:

try:
    val = dic[key]
except KeyError:
    do_some_stuff()
else:
    do_some_stuff_with_val(val)

下面是我喜欢使用这种模式的另一个地方:

 while data in items:
     try
        data = json.loads(data)
     except ValueError as e:
        log error
     else:
        # work on the `data`

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/

我发现其他有用的处理可能不正确的配置文件:

try:
    value, unit = cfg['lock'].split()
except ValueError:
    msg = 'lock monitoring config must consist of two words separated by white space'
    self.log('warn', msg)
else:
     # get on with lock monitoring if config is ok

读取锁配置的异常禁用锁监控，ValueErrors记录有用的警告消息。

可以在finally子句中以通用的方式使用此构造来处理异常，而在没有异常时执行其他操作:

class TooManyRetries(RuntimeError):
    pass


n_tries = 0
max_retries = 2
while True:
    try:
        n_tries += 1
        if n_tries >= max_retries:
            raise TooManyRetries
        fail_prone_operation()
    except Exception1 as ex:
        # handle1
    except Exception2 as ex:
        # handle2
    except Exception3 as ex:
        # handle3
    except TooManyRetries as ex:
        raise
    else: # No exception
        n_tries = 0
    finally:
        common_restore_state()
        continue