map float string '0.0' puzzle

[Brian Quinlan]

> >>> row = ('0.0', '1.0', None)
> >>> map(lambda setting: setting and float(setting) or None, row)
 
> Specifically, why is the return value of the first map operation:
> [None, 1.0, None]
> 
> I was expecting:
> [0.0, 1.0, None]

Can you explain why you were expecting that? The expression is:

[('0.0' and 0.0) or None, ('1.0' and 1.0) or None, (None and ?) or None)]
             ^ False              ^ True             ^ False     


You can look at and/or like this (except that function calls don't do
short-circuit evaluation):

def and(*args):
    for a in args:
         if not a:
             return a
    return args[-1]

def or(*args):
    for a in args:
        if a:
            return a
    return args[-1]

So you have:

or(and(settings, float(setting)), None)

Cheers,
Brian