Allow configuring Jinja with StrictUndefined

[yajo]

Discussed in https://github.com/orgs/copier-org/discussions/1512

^{Originally posted by coretl February 9, 2024}
I've been bitten a couple of times by this workflow:

• Change the name of a variable in copier.yml
• Forget to rename some of the references to that variable in my template
• End up with that variable being rendered blank in a text file somewhere when using the template

It would appear that Jinja does this by default, and the way it suggests to raise an error is by supplying undefined=StrictUndefined in its Environment. I tried this in copier.yml:

_envops:
    undefined: StrictUndefined

But it fails because it can't turn the string into a callable:

  File "/venv/lib/python3.11/site-packages/copier/main.py", line 1024, in run_copy
    with Worker(src_path=src_path, dst_path=Path(dst_path), **kwargs) as worker:
  File "/venv/lib/python3.11/site-packages/copier/main.py", line 205, in __exit__
    raise value
  File "/venv/lib/python3.11/site-packages/copier/main.py", line 1025, in run_copy
    worker.run_copy()
  File "/venv/lib/python3.11/site-packages/copier/main.py", line 745, in run_copy
    self._ask()
  File "/venv/lib/python3.11/site-packages/copier/main.py", line 421, in _ask
    jinja_env=self.jinja_env,
              ^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/functools.py", line 1001, in __get__
    val = self.func(instance)
          ^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/copier/main.py", line 500, in jinja_env
    env = SandboxedEnvironment(
          ^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/jinja2/sandbox.py", line 253, in __init__
    super().__init__(*args, **kwargs)
  File "/venv/lib/python3.11/site-packages/jinja2/environment.py", line 366, in __init__
    _environment_config_check(self)
  File "/venv/lib/python3.11/site-packages/jinja2/environment.py", line 126, in _environment_config_check
    assert issubclass(
           ^^^^^^^^^^^
TypeError: issubclass() arg 1 must be a class

Is there a way to make the copier copy command fail if you use an undefined name in a template?

Related to #313 but with a different aim in mind

[coretl]

Would you like a PR for this?

[yajo]

Of course.

[evalott100]

I'll start working on this now.