Fix default value for flags with multiple=True

[amyreese]

Sets a default value (empty tuple) for flags with multiple=True. Adds a test case (that fails on 8.1.2) to help prevent future regression.

Fixes #2246

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[amyreese]

I'm happy to add the changes documentation, just wasn't sure what wording would be preferred for that.

[davidism]

Looking at this example, I'm not sure this was ever intended? You should use count=True if you want a flag without value that can be specified multiple times.

[amyreese]

Looking at this example, I'm not sure this was ever intended? You should use count=True if you want a flag without value that can be specified multiple times.

Hmm, I was just trying to test count, and got this error message:

TypeError: 'count' is not valid with 'is_flag'.

Perhaps it would be better to explicitly disallow is_flag=True and multiple=True in the same manner, rather than waiting for a vague error deep in the parsing code path.

[davidism]

Yes, that seems better.

[amyreese]

Opened #2248 with alternative change.

[davidism]

Well, in the end I decided to go with this solution instead, see #2550 😅