Add pre-commit

The way that pre-commit installs tools via git tags and ignores the
environment in use is neat but kind of annoying, since it means either
duplicating them, only using the tool via pre-commit, or bypassing the
typical setup with a "system" pre-commit configuration. For ruff I've
removed it from the venv and can only use it via pre-commit, which seems
fine since I only use it in simple ways, but could become annoying
later.

In the case of mypy I immediately ran into a problem with the fact that
I would need to specify my dependencies (or type stubs for them) in the
pre-commit configuration to get it to work correctly in the isolated
environment. I absolutely don't want that duplication, so instead I'm
using the "system" workaround, which isn't much extra configuration.
This follows the logic described here:
python/mypy#13916. I also disabled
pass_filenames due to it not making much sense with mypy, as described
there.

Another pain point is needing to have pre-commit available to commit,
which right now means activating the appropriate environment. Probably a
case where pipx would be ideal, but I don't feel like setting it up
right now.

.pre-commit-config.yaml

@@ -0,0 +1,26 @@
+repos:
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v4.6.0
+  hooks:
+  - id: check-added-large-files
+  - id: check-merge-conflict
+  - id: check-toml
+  - id: check-yaml
+  - id: end-of-file-fixer
+- repo: https://github.com/astral-sh/ruff-pre-commit
+  rev: v0.6.6
+  hooks:
+  - id: ruff
+    args: [ --fix ]
+  - id: ruff-format
+# Following recommendations from https://github.com/python/mypy/issues/13916
+# This does make the mypy step slower
+- repo: local
+  hooks:
+  - id: mypy
+    name: mypy
+    language: system
+    entry: mypy
+    types_or: [python, pyi]
+    args: ["--strict", ""]
+    pass_filenames: false

README.md

@@ -52,16 +52,30 @@ After activating an environment with dev dependencies:
 pytest tests/
 ```
 
-### Format and lint
+### Formatting, linting, and type-checking
+Formatting and linting are primarily done using ruff, and managed via pre-commit.
+Type-checking is done using mypy, which is installed in the venv (outside of
+pre-commit's isolated venv) and run by pre-commit (for reasons see
+https://github.com/python/mypy/issues/13916).
+
+Set up pre-commit:
 ```
-ruff format && ruff check
+pre-commit install
 ```
-To autofix linting errors (where possible):
+
+Checks will run before commit, and abort commit if any fail or make changes - in which
+case the changes will need to be reviewed and git added.
+
+To run ruff manually on all files:
 ```
-ruff check --fix
+pre-commit run ruff --all-files
 ```
 
-### Static type check
+To run mypy manually on all files:
+```
+pre-commit run mypy --all-files
+```
+or
 ```
 mypy --strict .
 ```

pyproject.toml

@@ -14,13 +14,14 @@ dependencies = [
 
 [project.optional-dependencies]
 dev = [
+    "mypy", # Installed here but used via pre-commit
     "pip-tools",
+    "pre-commit",
     "pytest",
     "pytest-mock",
-    "ruff",
-    "mypy",
 ]
 
+# Ruff is configured here but installed via pre-commit
 [tool.ruff.lint]
 select = [
     "A",    # flake8-builtins
@@ -69,4 +70,4 @@ ignore = [
 "tests/*" = [
     "ARG002",   # Pytest fixtures often require unused arguments
     "PLR2004",  # Magic values in tests are fine
-]
+]

requirements-dev.txt

@@ -8,10 +8,18 @@ build==1.2.2
     # via pip-tools
 certifi==2024.8.30
     # via requests
+cfgv==3.4.0
+    # via pre-commit
 charset-normalizer==3.3.2
     # via requests
 click==8.1.7
     # via pip-tools
+distlib==0.3.8
+    # via virtualenv
+filelock==3.16.1
+    # via virtualenv
+identify==2.6.1
+    # via pre-commit
 idna==3.8
     # via requests
 iniconfig==2.0.0
@@ -20,14 +28,20 @@ mypy==1.11.2
     # via wordall (/home/lewis/repos/wordall/pyproject.toml)
 mypy-extensions==1.0.0
     # via mypy
+nodeenv==1.9.1
+    # via pre-commit
 packaging==24.1
     # via
     #   build
     #   pytest
 pip-tools==7.4.1
     # via wordall (/home/lewis/repos/wordall/pyproject.toml)
+platformdirs==4.3.6
+    # via virtualenv
 pluggy==1.5.0
     # via pytest
+pre-commit==3.8.0
+    # via wordall (/home/lewis/repos/wordall/pyproject.toml)
 pyproject-hooks==1.1.0
     # via
     #   build
@@ -38,14 +52,16 @@ pytest==8.3.3
     #   wordall (/home/lewis/repos/wordall/pyproject.toml)
 pytest-mock==3.14.0
     # via wordall (/home/lewis/repos/wordall/pyproject.toml)
+pyyaml==6.0.2
+    # via pre-commit
 requests==2.32.3
     # via wordall (/home/lewis/repos/wordall/pyproject.toml)
-ruff==0.6.4
-    # via wordall (/home/lewis/repos/wordall/pyproject.toml)
 typing-extensions==4.12.2
     # via mypy
 urllib3==2.2.2
     # via requests
+virtualenv==20.26.5
+    # via pre-commit
 wheel==0.44.0
     # via pip-tools