How to check you haven't used an undefined variable in the template?

[coretl]

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

[yajo]

Copier is lacking this feature, but it seems like a good idea. I opened #1522 to track it.

[object-Object]

It would be great for this to be supported by Copier, but here are a couple of workarounds in the meantime.

1: Monkeypatch Worker

This method only applies StrictUndefined when the patched script is used (ie. during development), not when normally copying from the template; this means that end users won't have to install any extra packages or change how they use the template.

Create a file (eg. copier_strict.py) with these contents, and run that script instead of copier (eg. python -m copier_strict copy path/to/project/template path/to/destination). For programmatic use, StrictUndefinedWorker.run_copy could be used directly (basically reimplementing copier.run_copy) instead of patching Copier.

from functools import cached_property
import copier.main
from jinja2 import StrictUndefined

class StrictUndefinedWorker(copier.main.Worker):
    @cached_property
    def jinja_env(self):
        env = super().jinja_env
        env.undefined = StrictUndefined
        return env

copier.main.Worker = StrictUndefinedWorker

if __name__ == "__main__":
    from copier.cli import CopierApp

    CopierApp.run()

2: Custom extension

This method requires you to create a Python package that end users can install, since it uses the _jinja_extensions option.

Create a package with these contents, install it, and add your_package.StrictUndefinedExtension to _jinja_extensions in your copier.yml file. Tell your end users to install this package before copying the template.

from jinja2 import Environment, StrictUndefined
from jinja2.ext import Extension

class StrictUndefinedExtension(Extension):
    def __init__(self, environment: Environment):
        super().__init__(environment)
        environment.undefined = StrictUndefined