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[Python-Dev] PEP 572 and assert


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