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

Tim Peters tim.peters at gmail.com
Tue Jul 17 12:25:05 EDT 2018

[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 ;-) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20180717/70824815/attachment.html>

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