Load template extensions by class to prevent import deadlock

[frenck]

Breaking change

Proposed change

The template integration fails to load on startup with a DeadlockError in the MathExtension Jinja2 extension. This happens because template extensions are registered by import path string (e.g., "homeassistant.helpers.template.extensions.MathExtension"), which causes Jinja2 to call import_module() at runtime. When multiple threads create a TemplateEnvironment simultaneously during startup, the Python import lock deadlocks.

This changes the extension loading to import the classes at module level and pass them directly to add_extension(), avoiding runtime imports entirely. The existing jinja2.ext extensions remain as strings since those are external and always pre-loaded.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes template DeadlockError extensions math #170975
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.
• Any generated code has been carefully reviewed for correctness and compliance with project standards.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies a diff between library versions and ideally a link to the changelog/release notes is added to the PR description.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[Copilot]

Pull request overview

Fixes a startup deadlock in the template integration by avoiding Jinja2 runtime imports of Home Assistant’s template extensions, which could deadlock when multiple threads create TemplateEnvironment instances concurrently.

Changes:

• Import Home Assistant template extension classes at module import time.
• Register Home Assistant extensions with TemplateEnvironment.add_extension() by passing the extension classes directly instead of import-path strings.