Is it possible to calculate default values dynamically?

[pfmoore]

I want to prompt the user for their name, email and github username, but I'd like to provide sensible defaults (which can be obtained by running git config to get the appropriate values). Is it possible to create defaults dynamically like this?

[pawamoy]

I suppose Copier could retrieve the values itself and inject them in the Jinja context used for questions and answers. We could then set defaults with something like default: "{{ _git_user_name }}" or default: "{{ _git_user_email }}".

[pfmoore]

I guess so, but who would decide what was worth including? I was hoping there would be a way to execute code to generate defaults, as that's a bit more flexible (although it's also a security risk, so I'd be fine if the ability was limited somehow).

Is there a hook that templates can use to adjust the Jinja context? Or is this something a Jinja plugin could do? (I don't know much about Jinja plugins).

[pawamoy]

I have successfully written a Copier/Jinja extension that can be used to alter the rendering context (now in the org: https://github.com/copier-org/copier-templates-extensions). I'm not sure it would be possible to do the same thing for the prompt context. I'll need to check Copier's code flow 🙂

[pfmoore]

I just tested the extension I wrote (see the answer I added below) and that seems to work fine in the prompt context. Thanks for the comment, I wasn't even aware this could be an issue 🙂

[pfmoore]

Actually, having put the question that way, I realised that this is at least partially covered in the FAQ here. And in fact, writing a simple Jinja extension that just adds stuff to the environment isn't particularly hard:

from jinja2.ext import Extension


class TestExtension(Extension):
    def __init__(self, environment):
        super().__init__(environment)

        # add the defaults to the environment
        environment.globals["some_variable_name"] = "The value you want"

So I guess the answer is that to do this I can write a Jinja extension to do this, and load that extension in copier.yml.

Have I missed any subtleties here? As I said, I don't know much about Jinja plugins (the docs say "as long as no template was loaded it’s safe to modify this" - it seems to work here so I assume extensions are initialised early enough that we're OK).

It's a bit annoying to have to depend on an extension being installed, but the copier_templates_extensions approach seems to end up with the extension code being included in the generated project, which isn't ideal either. So I'll probably stick with a plain extension.

[yajo]

Have you tried with the shell extension?

It should let you do something like:

# copier.yaml
_jinja_extensions:
  - jinja2_shell_extension.ShellExtension
email:
  default: "{{ 'git config user.email'|shell }}"

[pfmoore]

Ooh, nice! I didn't know that existed. I did wonder if this was something that might already exist, but I couldn't find a good way of looking for it.

[yajo]

You have a hint in the docs (I used that one FWIW 😆).

[pfmoore]

Bah, I missed that. Sorry, I've just discovered copier in the process of trying to find a replacement for my current cookiecutter templates, and I have a bad habit of getting over-enthusiastic and jumping straight into complex stuff, without taking time to get properly familiar with the basics. Sorry about that and thanks for your patience and help 🙂

[yajo]

😄 don't worry, we're full of cookiecutter refugees here.

This might help you: https://copier.readthedocs.io/en/stable/comparisons/

Probably it's not complete, but PRs are always welcome!