Q from python tut...

[Daniel Faulkner]

Deryk Barker wrote:

> Gordon McMillan (gmcm at hypernet.com) wrote:
> : prasad writes:
> : >
> : > I am learning python using the standard tut that came with the
> : > distri. I know perl a little bit. Here are some of the things that I
> : > found confusing till section 5.5.
> : >
> : >
> : > val = '' or 'aa' or 'bb'
> : > val gets the value 'aa'
> : >
> : > shouldn't the expr '' or 'aa' or 'bb' evaluate to 0 or 1 and hence
> : > val getting the value of 0 or 1.
> :
> : Python has a protocol for determining whether an object considers
> : itself "true" or not. Collections (tuples, lists, dicts and
> : strings) are true if they're non-empty. The fact that val gets bound
> : to the first "true" thing in the RHS is very handy, if somewhat
> : surprising at first.
>
> And is a direct result of the short-circuit evaluation being used.
>
> on the basis that true or X =  true
>                   false or X = X
> and
>                   false and X = false
>                   false or X  = X
>
> As soon as you find true (false) when evaluating an or (and) then you
> can stop and not bother to evaluate any more of the expression.
>
> You need to watch for side-effects...
>
> Similarly any non-zero numeric value is true, and short-circuiut
> evaluation is also used, so that the value of
>
>         2 or 5
>
> is 2 whereas the value of
>
>         0 or 5
>
> is 5.
>
> --
> |Deryk Barker, Computer Science Dept. | Music does not have to be understood|
> |Camosun College, Victoria, BC, Canada| It has to be listened to.           |
> |email: dbarker at camosun.bc.ca         |                                     |
> |phone: +1 250 370 4452               |         Hermann Scherchen.          |


Or to make it more simple python detects that the first item is false and so
moves on to the next and uses that.