Handling 3 operands in an expression without raising an exception

[Νίκος]

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.

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.