Incompatibility With Python 3.14

[aholovin-smtc]

When running chart-testing (ct) on a self-hosted GitHub Actions runner that has Python 3.14 installed, CT creates its own virtual environment using Python 3.14.
This results in immediate failures during schema validation due to Yamale being incompatible with Python 3.14.

What Happens

CT bundles its own Python virtualenv under the toolcache, for example:
/var/lib/github-action-runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14

When Python 3.14 is detected, CT installs a venv using that version.
The bundled Yamale version used by CT still relies on deprecated ast.Num and other removed AST nodes, which leads to errors such as:

Yamale / Python 3.14 error traceback

Linting chart "base-service => (version: "0.0.0", path: "base-service")"
Validating /var/lib/github-action-runner/runner/_work/helm-charts/helm-charts/base-service/Chart.yaml...
Traceback (most recent call last):
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/bin/yamale", line 7, in <module>
  sys.exit(main())
           ~~~~^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/command_line.py", line 124, in main
  _router(args.path, args.schema, args.cpu_num, args.parser, not args.no_strict)
  ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/command_line.py", line 104, in _router
  _validate_single(root, schema_name, parser, strict)
  ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/command_line.py", line 69, in _validate_single
  _validate(s, yaml_path, parser, strict, True)
  ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/command_line.py", line 28, in _validate
  schema = yamale.make_schema(schema_path, parser)
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/yamale.py", line 15, in make_schema
  s = Schema(raw_schemas[0], path, validators=validators)
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/schema/schema.py", line 17, in __init__
  self._schema = self._process_schema(DataPath(),
                 ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
                                      schema_dict,
                                      ^^^^^^^^^^^^
                                      self.validators)
                                      ^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/schema/schema.py", line 36, in _process_schema
  schema_data[key] = self._process_schema(path + DataPath(key),
                     ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
                                          data,
                                          ^^^^^
                                          validators)
                                          ^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/schema/schema.py", line 40, in _process_schema
  schema_data = self._parse_schema_item(path,
                                        schema_data,
                                        validators)
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/schema/schema.py", line 47, in _parse_schema_item
  return syntax.parse(expression, validators)
         ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/syntax/parser.py", line 40, in parse
  _validate_expr(tree.body, validators)
  ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
File "/var/lib/github-action-runner/runner/_work/_tool/ct/3.12.0/amd64/venv/lib/python3.14/site-packages/yamale/syntax/parser.py", line 22, in _validate_expr
  ast.Constant, ast.Num, ast.Str, ast.Bytes, ast.NameConstant]
                ^^^^^^^
AttributeError: module 'ast' has no attribute 'Num'

Impact

• Chart validation fails consistently.
• CT is unable to perform schema validation.
• Pipelines depending on CT break once Python 3.14 is present on the runner.

Expected Behavior

Ideally, CT should:

• avoid selecting incompatible Python versions when creating its internal virtual environment, or
• update the bundled dependencies to support Python 3.14, or
• consider moving away from Python-based validation altogether.

Workaround

We temporarily reverted our environment to Python 3.12, which restores compatibility.

Requested Fix

It would be greatly appreciated if future versions of CT could:

• add support for Python 3.14, or
• ensure that the internal virtual environment is fully isolated from system Python version changes, or
• consider adopting a Python-independent approach for schema validation.

This would help avoid compatibility issues in environments where system Python versions evolve independently of CT.

[cpanato]

Are you willing to open a PR to fix the issue?

[aholovin-smtc]

Hello @cpanato
Yes, I could try to fix it.

[pierluigilenoci]

I created 23andMe/Yamale#279, 23andMe/Yamale#280 and #787

[pierluigilenoci]

@aholovin-smtc, which version of the chart-testing action do you use?