[Python-Dev] PEP 572 and assert (original) (raw)

MRAB python at mrabarnett.plus.com
Tue Jul 17 15🔞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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