[Python-Dev] PEP 572 and assert

Chris Angelico rosuav at gmail.com
Tue Jul 17 04:56:36 EDT 2018

On Tue, Jul 17, 2018 at 6:50 PM, Serhiy Storchaka <storchaka at gmail.com> wrote:
> 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?

Asserts with side effects are already a bad idea. PEP 572 makes no
change to this. If you're putting any sort of side effects inside
assertions, you're playing with fire.


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