Handling 3 operands in an expression without raising an exception

[Antoon Pardon]

Op 26-09-13 10:39, Νίκος schreef:
> Στις 26/9/2013 11:12 πμ, ο/η Jussi Piitulainen έγραψε:
>> Νίκος writes:
>>
>>> Σ�ις 26/9/2013 10:48 πμ, ο/η Jussi Piitulainen
>>> έγ�α�ε: > ί�ος writes:
>>>>
>>>>> How can i wrote the two following lines so for NOT to throw out
>>>>> KeyErrors when a key is missing?
>>>>>
>>>>> city = gi.time_zone_by_addr( os.environ['HTTP_CF_CONNECTING_IP'] ) or
>>>> ...
>>>>> I was under the impression that the 'or' operator was handling this
>>>>> in case one operand was failing but its not the case here.
>>>>
>>>> "f(x) or g(x)" raises an exception if "f(x)" raises an exception, or
>>>> if "f(x)" returns a false value and "g(x)" raises an exception.
>>>>
>>>>> Then i thought of os.environ.get() to default to something but then
>>>>> again we have 3 operand in the expression.
>>>>
>>>> Adapt this:
>>>>
>>>>     >>> {}.get('foo') or {'empty':''}.get('empty') or 'catchall'
>>>>     'catchall'
>>>>
>>>> Or nest the calls this way if an empty string is a valid value:
>>>>
>>>>     >>> {}.get('foo', {'empty':''}.get('empty', 'catchall'))
>>>>     ''
>>>>
>>>> This will compute the default values even when they are not used.
>>>
>>> I'am sorry but i do not understand the last statements at all so i
>>> can have chnace to adapt them.
>>
>> Do you know what {} is?
>>
>> Do you know what {}.get('foo') is?
>>
>> Do you know what x.get('foo') is if x is {}?
>>
>> Do you know what {'empty':''}.get('empty') is?
>>
>> Do you know what {'empty':''}.get('fruit') is?
>>
>> Do you know what (None or '' or 'catchall') is?
>>
>> Do you know what {}.get('foo', 'bar') is?
>>
>> Do you know what {}.get('foo', {}.get('bar', 'huh')) is?
>>
>> Do you know what ('foo'[3] or 'else') does?
>>
>> Do you know what ('foo' or 'else'[5]) does?
>>
>> Do you know how to launch an interactive Python session where you can
>> play with such expressions until you get the hang of it? There is no
>> substitute for that experience.
>>
>> Do you know that you can ask for help({}.get) or help(dict.get) or
>> even help(os.environ.get) during such an interactive Python session,
>> and Python (unlike Macbeth's spirits from the vasty deep) will answer?
>>
> You dont have to be ironic. I dont have the experience you do.

That is irrelevant. For the responsibility you have taken upon yourself,
it seems you should have experience enough to figure this out. If you
can't, you just are not up for the job.

> Up until now i have this:
> 
> city = gi.time_zone_by_addr( os.environ['HTTP_CF_CONNECTING_IP'] ) or
> gi.time_zone_by_addr( os.environ['REMOTE_ADDR'] ) or "Άγνωστη Πόλη"
> 
> 
> can this be written as:
> 
> city = gi.time_zone_by_addr( os.environ.get('HTTP_CF_CONNECTING_IP',
> os.environ['REMOTE_ADDR'] )) or "Άγνωστη Πόλη"
> 
> It makes it more easily for me to understand this way.

Experiment and find out for yourself. That is the only way you will
acquire the experience and understanding you need. Sure we could
spoon feed you the line you need, but that will only result in you
using that line without any understanding of what you are doing so
that if in a few months time something goes wrong, you again will
have no understanding of what goes on and still will have no clue
on how to handle a problem.

-- 
Antoon Pardon