[Tutor] 'or' in assignment (not if statement)?

[Hugo Arts]

On Fri, Dec 10, 2010 at 2:07 AM, Alan Gauld <alan.gauld at btinternet.com> wrote:
>
> "Alex Hall" <mehgcap at gmail.com> wrote
>>
>> val=val or 1
>
>> I am guessing that val is an int. If val==0, the 'or' kicks in and
>> val=1, else the or is not needed and val=val. Am I close?
>
> Yes this is a combination of what is known as short circuit evaluation of
> boolean expressions and a quirk of Python that returns the actual value of
> something that is being treated as a boolean.
>

Doesn't short-circuit evaluation refer specifically to the behavior
where arguments are only evaluated if they need to be? It's a very
useful feature, but not technically required for the "val = val or 1"
behavior to work.

Also, returning on of its operands rather than a boolean is hardly a
quirk, since basically all dynamic languages do it ever since perl
made "val = val or 1" an idiom (at least, I think it was perl).

There is more to the innocent little or. See
http://docs.python.org/library/stdtypes.html#truth-value-testing
It works on any type, not just numbers, and operates by truth value
testing, which is more complicated than it might first seem.

Hugo