Use templated default during update

[adrien-berchet]

If I have a template with something like:

a_value:
  type: str
  choices:
    - a
    - b
  default: a
another_value:
  type: str
  default: "{%- if a_value == 'a' -%}CASE A{%- else -%}CASE B{% endif %}"

I would like to create the following workflow:

1. create a new project using the template: copier the_template the_project and with the following answers:
   1. a_value = a
   2. use the templated default value: another_value = CASE A
2. update the projec: cd the_project; copier update and with the following answers:
   1. a_value = b (so we change the value)
   2. here if the previous answer was equal to the default value, then I want to change it to the new default value (i.e. CASE B in this example), else we keep the given custom value. In both cases the user can still change the value by answering the question if he would like.

Is it possible?

To be more specific, I would like to use this workflow to migrate repositories from Gitlab to Github. In my copier.yml I have a destination value that can be equal to either github or gitlab and then I have another repo_url value which is templated according to the destination. When I update the project, I would like to change the URL automatically if it was set to the default value (but keep the current value if it was customized when the project was generated) before the prompt ask the new value to the user, so the user already has the correct default value in the prompt.

I tried to use https://github.com/copier-org/copier-templates-extensions#context-hook-extension but I could not find a way to achieve this.

Thanks!