On 2018年07月17日 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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