Handling 3 operands in an expression without raising an exception

[Jussi Piitulainen]

Νίκος writes:

> ΣÏις 26/9/2013 10:48 πμ, ο/η Jussi Piitulainen
> έγÏαÏε: > ίºÎ¿Ï‚ 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?