Forbid shlex.quote returning a quoted empty string (`”””`) for falsey non-`str` data

Bug report

Bug description:

Full discussion and history of the bug is on discuss.python.org.

tldr:

shlex.quote's opening check for the empty string as a special case is still placed before the isinstance type check. This is intentional as when the isinstance check was added, the patch was intended to not break compatibility with eg people expecting shlex.quote(None) to return "''", implicitly coercing None to the empty string "" and then quoting it as "''".

def quote(s):
    if not s:
        return "''"

    if not isinstance(s, str):
        raise TypeError(f"expected string object, got {type(s).__name__!r}")
    ...

We think either of the following is a best course of action:

• Print a DeprecationWarning when one calls quote on falsey non-str data
• Leave the function alone, as people may now be relying on this as a feature, not a bug, and we don't gain much by doing a patch.

Or we could do a hard-break and just swap the order of the isinstance and if not s checks around, which would break anybody relying on None -> "''" (not recommended!).

I have a couple of git tags that I will link below with reference implementation for 'fixing' this issue (if it needs to be fixed). I just need the gh- number of this issue first for the blurb entries.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Read shlex.quote's current empty-string and type checks, then review the linked discussion and compatibility history. The issue is not ready for a concrete implementation: maintainers must choose whether to warn, preserve behavior, or hard-break before completion can be defined.