Handling 3 operands in an expression without raising an exception

[Νίκος]

Στις 26/9/2013 10:48 πμ, ο/η Jussi Piitulainen έγραψε:
> Νίκος 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.