Handling 3 operands in an expression without raising an exception

[Νίκος]

Στις 26/9/2013 11:46 πμ, ο/η Νίκος έγραψε:
> Στις 26/9/2013 11:39 πμ, ο/η Νίκος έγραψε:
>> 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.
>
> Let me try be more specific:
>
> i want to switch 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"
>
> host = socket.gethostbyaddr( os.environ['HTTP_CF_CONNECTING_IP'] )[0] or
> socket.gethostbyaddr( os.environ['REMOTE_ADDR'] )[0] or "Άγνωστη Προέλευση"
>
> because it raises KeyError exceptions to this:
>
> city = gi.time_zone_by_addr( os.environ.get('HTTP_CF_CONNECTING_IP',
> 'REMOTE_ADDR') )    or "Άγνωστη Πόλη"
>
> host = socket.gethostbyaddr( os.environ.get('HTTP_CF_CONNECTING_IP',
> 'REMOTE_ADDR') )[0] or "Άγνωστη Προέλευση"
>
> But that doesnt seem to also work.
> I want to use the get method because i know if it doesnt does detect a
> dictionary key item than default back what we give it.

Can you please tell me why my alternative fails to work although i'am 
using the .get method to default to something?