Swap mypy in favour of ty

[paddyroddy]

ty is a fast alternative to mypy. Although it has been released for a while now, the pre-commit hook has only just been released. We've used this successfully in GLASS glass-dev/glass#1133 and I've used it in SLEPLET astro-informatics/sleplet#489. It is being rapidly developed so will only get better. I know Astral sold out, but I doubt we're going to move from uv realistically.

[paddyroddy]

I am using https://github.com/j178/prek in favour of pre-commit locally. I found that running on the top level was running the hooks in tests/data/test_package_generation which was causing bugs.

[paddyroddy]

@pre-commit.ci seems like it's having a mare

error: Request failed after 3 retries in 6.8s
  Caused by: Failed to fetch: `https://pypi.org/simple/pytest-venv/`
  Caused by: error sending request for url (https://pypi.org/simple/pytest-venv/)
  Caused by: client error (Connect)
  Caused by: dns error
  Caused by: failed to lookup address information: Temporary failure in name resolution

[Copilot]

Pull request overview

This PR updates the cookiecutter template (and its test fixture) to use Astral’s ty type checker instead of mypy, and wires ty into pre-commit so type checking runs consistently in local development and CI.

Changes:

• Replace the mirrors-mypy pre-commit hook with astral-sh/ty-pre-commit across the root config, template, and test fixture.
• Add dependency-groups.typechecking and tool.ty configuration to support isolated type-check runs.
• Update documentation to recommend ty (while still listing mypy as an alternative).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/data/test_package_generation/pyproject.toml	Updates fixture dependencies/config from mypy to ty and adds typechecking group/config.
tests/data/test_package_generation/.pre-commit-config.yaml	Switches fixture pre-commit type checking hook from mypy to ty.
pyproject.toml	Adds a typechecking dependency group and ty overrides for this repository.
docs/pages/tutorial.md	Updates recommended tooling to reference ty instead of mypy.
docs/pages/linting.md	Promotes ty as the preferred type checker and downgrades mypy to “Good”.
docs/pages/libraries/jupyter-notebooks.md	Updates notebook linting guidance to reference ty instead of mypy.
{{cookiecutter.project_slug}}/pyproject.toml	Updates template dev dependencies/config from mypy to ty and adds typechecking group/config.
{{cookiecutter.project_slug}}/.pre-commit-config.yaml	Switches template pre-commit type checking hook from mypy to ty.
.prekignore	Adds ignore rules for prek users (separate from pre-commit’s own excludes).
.pre-commit-config.yaml	Switches this repo’s pre-commit type checking hook from mypy to ty.

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[samcunliffe]

Looks good to me! Nice and complete. Website looks correct too.

Is this a two-reviewers jobby? Pinging someone else from @UCL-ARC/collaborations-python-tooling

[samcunliffe]

Looks good to me! Nice and complete. Website looks correct too.

Is this a two-reviewers jobby? Pinging someone else from @UCL-ARC/collaborations-python-tooling

Having said that it's been open for more than 1 month. So I'd say any objectors would have made themselves known by now.

[matt-graham]

From my perspective also happy for this to go in. Agree that the additional jeopardy of recommending ty given Astral ownership situation is minimal if we are already recommending uv. Have added a small suggestion to record the clarification about .prekignore file added in your review comment as a comment in file (seems like from documentation .prekignore follows .gitignore syntax and so should support # comments).

[paddyroddy]

@pre-commit.ci seems like it's having a mare

error: Request failed after 3 retries in 6.8s
  Caused by: Failed to fetch: ``https://pypi.org/simple/pytest-venv/``
  Caused by: error sending request for url (https://pypi.org/simple/pytest-venv/)
  Caused by: client error (Connect)
  Caused by: dns error
  Caused by: failed to lookup address information: Temporary failure in name resolution

Just realised our very own @dstansby made an issue on this pre-commit-ci/issues#249

[paddyroddy]

I'm going to merge. Hopefully that @pre-commit-ci either fixes itself. If not we can raise a new issue to tackle it.