[Python-Dev] PEP 572 and assert

MRAB python at mrabarnett.plus.com
Tue Jul 17 15:18:51 EDT 2018

On 2018-07-17 19:34, Tim Peters wrote:
> [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.
Why use len(...) == 0 instead of not(...)?

     assert not(subdirs := list(path.iterdir())), subdirs

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