subprocess: `CalledProcessError.__str__` crashes when returncode is None

Calling str() on a CalledProcessError crashes if its returncode is None.

The __str__ method uses if self.returncode and self.returncode < 0 for the "died with a signal" case, and otherwise formats the return code with %d. When returncode is None, that first check is falsy, so it falls through to the %d branch, and %d can't format None, so it raises TypeError. Having the exception's own string representation blow up is a pretty bad way to fail.

import subprocess
err = subprocess.CalledProcessError(None, "cmd")
str(err)  # TypeError: %d format: a real number is required, not NoneType

The fix is to check for returncode is None before the %d branch and return a plain message instead.

Found while going through devdanzin's audit of the standard library, item 9: https://gist.github.com/devdanzin/3198710e3c0128fda5e0a7b4e0768e5f

Linked PRs

• gh-153971
• gh-155923
• gh-155924
• gh-155931

Commencez par subprocess.CalledProcessError.str et reproduisez l’exemple de l’issue avec un None returncode. Ajoutez une couverture pour la représentation qui échoue et confirmez que str(err) renvoie un message simple sans lever d’exception ; consultez les PRs liés avant de commencer, car le travail est déjà associé à cette issue.