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Tue Jul 17 14:34:55 EDT 2018 · https://mail.python.org/pipermail/python-dev/2018-July/154683.html

[Barry Warsaw]

> Thanks!  I thought it was cute.  It was just something that occurred to me
> as I was reviewing some existing code.  The intent wasn’t to use `subdirs`
> outside of the assert statement, but I’m warm to it because it means I
> don’t have to do wasted work outside of the assert statement, or repeat
> myself in the assert message part.
>

Because the latter ("repeat myself") is probably more tempting, I'll just
note that the "laziness" of using an assignment expression instead may well
have nudged you toward writing _better_ code too.

   assert len(subdirs := list(path.iterdir())) == 0, subdirs

Assuming the result of list(path.iterdir()) can change over time (seems
very likely),

   assert len(list(path.iterdir())) == 0,  list(path.iterdir())

_could_ end up both triggering and displaying an empty list in the
exception detail.  The assignment-expression version cannot.
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[Python-Dev] PEP 572 and assert