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[Python-Dev] Examples for PEP 572

Steven D'Aprano steve at pearwood.info
Wed Jul 4 14:21:46 EDT 2018 
On Wed, Jul 04, 2018 at 08:33:17PM +0300, Ivan Pozdeev via Python-Dev wrote:


> >>while total != (total := total + term):
> >>    term *= mx2 / (i*(i+1))
> >>    i += 2
> >>return total

[...]
> It took me a few minutes to figure out that this construct actually 
> checks term == 0.

That's badly wrong.

Hint: it's floating point code. total and term are floats, so things 
like this are possible:

py> total = 9.5e95
py> term = 1.2
py> total + term == total
True

As you can see, term is nothing even close to zero.


> So, this example abuses the construct to do something it's not designed 
> to do: perform an unrelated operation before checking the condition.

Well, that's one opinion.


> (Cue attempts to squeeze ever mode code here.) I would fail it in review.

Okay.



-- 
Steve
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