🐛 Ensure that the value of is_flag is passed correctly to Click

[iurisilvio]

Discussion in #873.

Click supports optional value.

To make it work, you have to set is_flag=False, flag_value="any value", then:

• --foo return defined flag_value
• --foo something return something

It was broken because is_flag was passed as None when parameter_info.is_flag was explicitly False.

[svlandeg]

Thanks for this contribution! The discussion thread you opened in #873 clearly highlights the issue and the fact that the current Typer behaviour is inconsistent with how Click does it. Typer doesn't have any documentation around these specific parameters so it's fair to assume it would/should behave in the same way as Click does.

I could confirm the erratic behaviour and failing unit test on master, and with this PR the issues are resolved. The unit test perfectly captures the problems. The fix is minimal and accurate.

As such, I think this PR looks good to merge. Thanks again! 🙏

[svlandeg]

Maybe one point to consider when Tiangolo reviews this, is the lack of Typer documentation for these settings. Perhaps we can add a separate page somewhere, explaining how these arguments work and relate to eachother. We could potentially do this in follow-up work though.