[Tutor] "x and y" means "if x is false, then x, else y"??

[Richard D. Moores]

On Mon, Jul 5, 2010 at 00:55, Steve Willoughby <steve at alchemy.com> wrote:
> On 05-Jul-10 00:27, Richard D. Moores wrote:
>>
>>
>> See<http://docs.python.org/py3k/library/stdtypes.html#boolean-operations-and-or-not>.
>> I am quite familiar with the meaning of "x and y" in Python, and how
>> it is evaluated -- first x, and only if x is False, then evaluate y.
>> But I just can't read "if x is false, then x, else y" that way. In
>> fact, I can't read it at all. Is this a mistake in the Python 3 docs?
>> If not, can someone tell me how to make sense of it?

> Yes, in fact this was a common idiom before Python picked up the (x if p
> else y) syntax, and something of this nature is still commonplace in the
> Perl world (even though it has the ?: operator anyway).
>
> You already know about the "short circuiting" effect of "and" and "or",
> which affects whether the right argument is even evaluated at all, but the
> other piece of this puzzle is that the _return value_ of the expression is
> not a pure Boolean value of True or False, but is in fact either the value x
> or y itself.
>
> So in the case of
>    x and y
> if x is true, then we need to evaluate y, in which case y is returned. If y
> happened to be true, then that means both x and y were true, and hence the
> entire expression is true.  The particular "true" value we return here
> happens to be the (true) value of y.
>
> If y is false, then returning it yields the correct false value for the
> whole expression, although the actual (false) value of y is what we return.
>
> If x if false, then we need go no further and simply return x.
>
> So.
>
>
> 1 and 2  ==>  2    (true and true ==> true)
> 0 and 5  ==>  0    (false and true ==> false)
> 'hello' and '' ==> '' (true and false ==> false)
> 'xx' and 'yy' ==> 'yy' (true and true ==> true)
>
> x and y  ==>  x if x is false, else y
>
>
> Some people like using this to set values to defaults if no (true) value was
> input, in a semantically pleasing manner, thus:
>
> def __init__(self, a=None, b=None):
>  self.a = a or 123
>  self.b = b or 456

Steve,

Your answer seems very well-formulated. However, I've read it over and
over, but I keep getting hung up over the meaning of "the return
value" of an expression. I am of course familiar with values returned
by a function, but don't quite grasp what the return value of, say,
the y of "x and y" might mean.
Also, you distinguish between a return value of True and and the value
of y being such (say 5, and not 0) that it makes y true (but not
True). So another  thing I need to know is the difference between True
and true.  Also between False and false. And why the difference is
important.

I'm thinking that possibly what would help would be to contextualize
"x and y" in some snippet of code. I'm sorry to trouble you with my
denseness.

Dick