[Serhiy Storchaka]

Recently Barry shown an example:

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

It looks awful to me. It looks even worse than using asserts for
validating the user input. The assert has a side effect, and it depends
on the interpreter option (-O). Even if subdirs is not used outside of
the assert *now*, it is easy to introduce an error later, and it is hard
to notice it if tests are not ran with the -O option regularly.

Does PEP 572 encourages writing such code, discourages this, or
completely forbids?

The body of the PEP specifies semantics. My Appendix A gives some _opinions_ about "good" and "bad" uses, which boil down to "if it's not obviously at least a little win, don't use it".

I can't really guess whether the above is an obvious win or not without context. It is a win (to my eyes) if the code it replaced was

if __debug__:

subdirs = list(path.iterdir())

assert len(subdirs) == 0, subdirs

in which case the semantics are the same in either spelling, with or without -O, but the spelling at the top is less annoying ;-)