style query: function attributes for return codes?

[Reinhold Birkenfeld]

holger krekel wrote:
> Hi George, 
> 
> [george young Fri, Dec 10, 2004 at 10:45:47AM -0500]
>> [python 2.3.3, x86 linux]
>> I recently found myself writing something like:
>> 
>> def get_connection():
>>     if tcp_conn():
>>         if server_allows_conn():
>>             return 'good_conn'
>>         else:
>>             return 'bad_auth'
>>     else:
>>         return 'no_server'
>> 
>> cn = get_connection()
>> if cn == 'good_con': ...
>> 
>> 
>> This is obviously just evil, since a misspelling in the string
>> return is treacherous.
> 
> Right. 
> 
> I usually like to look at such problems from the angle of 
> what is most convenient for the *caller* side? 
> 
> And having to adress function attributes does
> not seem convenient.  I'd probably like to do from 
> the caller side something like: 
> 
>     conn = get_connection() 
>     if conn.good: 
>         ... 
>     elif conn.badauth: 
>         ... 
>     elif conn.noserver: 
>         ... 
> 
> which allows you to freely choose and hide your actual
> implementation at the "called" side. Example: 
> 
>     class Connection(object): 
>         def __init__(self, **kw): 
>             for name in kw: 
>                 assert name in ('good', 'badauth', 'noserver'), name 
>                 setattr(self, name, kw[name]) 
> 
>     def get_connection():
>         if tcp_conn():
>             if server_allows_conn():
>                 return Connection(good=True) 
>             else:
>                 return Connection(badauth=True) 
>         else:
>             return Connection(noserver=True) 

That's evil, because "if conn.good" raises an AttributeError instead of
evaluating to False if the connection is not good. You would have to
inizialize all three attributes in every construction of a Connection
object.

Reinhold


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