New strategies for conditional defaults on skipped questions with v8?

[tlambert03]

I have a template that makes heavy use of when conditionals along with defaults that also depend on prior questions. For example, take the following config:

mode:
  help: "Welcome! Please select a mode:"
  type: str
  default: full
  choices:
    Fully featured: full
    Simple package: simple
    Customize: customize

use_some_feature:
  when: "{{ mode == 'customize' }}"
  type: bool
  help: Would you like to use this specific feature?
  default: "{{ mode != 'simple' }}"

Prior to v8 the value of use_some_feature would have been:
- True (but not shown) if they selected "Fully featured" mode
- False (but not shown) if they selected "Simple" mode
- shown as a question if they selected "Customize" mode

I had thought that that was a well-supported feature due to the wording of the docs:

when: Condition that, if false, skips the question.
If a question is skipped, its answer will be:

• The default value, if you're generating the project for the 1st time.
• The last answer recorded, if you're updating the project.

As of v8, however, use_some_feature will always be False, regardless of whether the use choses "Full" or "Simple" mode.

This change is due to #1126. It was raised as a bug in #1175, but closed and stated as "backwards compatible". https://github.com/orgs/copier-org/discussions/678#discussioncomment-2822746 is also linked and #629 is linked for some "ideas".

After some reading though, I've still been unable to come up with a strategy that preserves the behavior mentioned above after v8.

I would greatly appreciate any tips (cc @sisp @pawamoy) for achieving the desired behavior, which again, is to have use_some_feature be:

• True (but not shown) if they selected "Fully featured" mode
• False (but not shown) if they selected "Simple" mode
• shown as a question if they selected "Customize" mode

[sisp]

I think you have a point here. #1126 isn't breaking for when: false when this is a hardcoded boolean literal because this abuse of a question for constructing a computed value isn't supported by Copier. But you're right that #1126 changed the behavior for a proper use of when: "{{ ... }}" in a breaking way. 😞 #1126 was the result of #958 raising a Question "<name>" is required error for a question without a default value whose when template evaluates to false because before #1126 the answer to a skipped question without a default value was None (carrying the semantics of "missing") which conflicts with #1075 (unless the question type happens to be json or yaml in which case None is still an improper value as the semantics is ambiguous). So #1126 was intended to address skipped questions without a default value not raising Question "<name>" is required, but you discovered that I also changed the (documented) behavior of skipped questions with a default value, and I believe that's not necessary to solve the original problem that led to #1126. The evolution and interactions of the different PRs are a bit complex, but I hope it's somewhat clear; and I believe the bottom-line is that you're right and I need to fix it.

Do you agree, @copier-org/maintainers?

[tlambert03]

thanks for the history/clarification! will wait to see what the rest of the maintainers think.

[pawamoy]

I agree that @tlambert03's use-case seems absolutely legit. I would even say that it's the only way where when makes sense. If it can only evaluate to true when there's a default, that's not right.

[yajo]

Have you tried using a macro?

[tlambert03]

Nope, but before we go down that discussion, can you weigh in on #1206? It does seem to me to be a pretty reasonable usage of when and default, so I'd like to know whether support for that is definitely going to be dropped before looking at macros

[tlambert03]

this is resolved by @sisp's work in #1220

[sisp]

More important is @yajo's careful assessment about the best implementation of this feature.