[Python-Dev] Examples for PEP 572

Steven D'Aprano steve at pearwood.info
Wed Jul 4 15:18:32 EDT 2018

On Wed, Jul 04, 2018 at 08:32:32PM +0200, Sven R. Kunze wrote:

> >>while total != (total := total + term):
> >>    term *= mx2 / (i*(i+1))
> >>    i += 2
> >>return total
> This very example here caught my eye.
> Isn't total not always equal to total? What would "regular" Python have 
> looked like?

Read the Appendix to the PEP:


And no, total is not always not equal to total. When total and term are 
sufficiently different, total+term underflows to just total, and the 
loop exits.

py> total = 1.5e30
py> term = 12.5
py> total + term != total

I read it as:

    while total != updated total:
        do stuff

and find it easier to follow than having to juggle the extra 
book-keeping "old" variable in the original code.



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