[Python-ideas] Fwd: quantifications, and tuple patterns

[Jim Jewett]

On Sat, Jan 14, 2012 at 9:01 PM, Steven D'Aprano <steve at pearwood.info> wrote:

>> On Sat, Jan 14, 2012 at 1:38 PM, Paul Moore <p.f.moore at gmail.com> wrote:

>>> Capturing a (quantification) "witness", which can be done using
>>> assignment-as-expression

> If I recall correctly, there have been occasional discussions about
> changing any() and all() to return the item found rather than a flag.

Even in this thread, there have been multiple posts that would shown
bugs when the witness itself had a boolean evaluation of False.

What if any and all started to return a Witness object that evaluated
to True/False, but made the value available?  I could see an object
like this being useful in other code as well.

    class Witness(object):
        def __bool__(self):  return self.truth
        def __init__(self, truth, value):
            self.truth=truth
            self.value=value

Of course, it might be nicer to inherit from type(True), and it still
doesn't quite solve the laziness problem with for, or the
assign-in-the-right-place problem with while.

-jJ