Across the codebase, handle the case a keyword argument is provided but set to None

[janluke]

In several points, Click relies exclusively on the following check to verify if an argument arg_name has been provided: arg_name in attrs. Then it assumes that attrs[arg_name] is not None. This PR is about treating None values in attrs as "not provided" too.

• Fixes Param decorators assume attrs[key] is not None just because key in attrs #1959 and similar issues in command decorators (see commit messages).

There are probably other points in the codebase that do the same thing. If you intend to merge this, I'll go on and search more in depth. I'll also add a test and a changelog entry.

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.

[davidism]

This seems fine, if you want to go through the code more like you mentioned.

I think what I'd really like is to stop using **kwargs, but it gets pretty verbose to write out all parameters for the subclass constructors and decorators. Would make IDEs and type checkers smarter. That can come later though.

[davidism]

That one seems to have broken a behavior.

Besides that change, were there any others? Let me know when this is ready for merge.

[janluke]

My last commit broke a test. This is due to the fact that default=None is not equivalent to not providing default for binary flags:

• when you don't provide default, default is set to False
• when default=None, default is set to None.

I think this may be accidental behavior but it's not senseless: settings default=None you can force the prompt to require an explicit value to be provided (it'll be [y/n] rather than [y/N]).

Nonetheless, I noticed that if you provide default=None to a flag without a secondary option, the prompt message is broken: it won't include "[y/n]" at the end.

@davidism I think it's better to handle this issue in another PR eventually. Thus, I'd revert my last commit here. What do you think?

[davidism]

Sounds good to me, I'll merge this one after you remove that commit.

[janluke]

I reverted the commit, I didn't remove it. This is ready to be merged.

Besides that change, were there any others? Let me know when this is ready for merge.

I searched for "in attrs" and "in kwargs". There are a few other places where Click relies on these but they seem legit.