Add typechecking tests using Mypy and Pytype

* New tasks are marked optional and will not stop CI even if they fail
* Added Makefile targets and GitHub Actions steps
* Added caching for `pip` in GitHub Actions workflow

.github/workflows/main.yml

@@ -47,6 +47,27 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
+          cache: 'pip'
 
       - name: Run tests
         run: make test VERBOSE=1
+
+      - name: Install Python deps for typechecking tests
+        run: python -m pip install -r requirements-test.txt
+
+      - name: Check types with Mypy
+        run: make mypy-test
+        # Mark this check optional for now, in case it starts out failing.
+        # We will make it required once we've fixed any existing issues.
+        continue-on-error: true
+
+      - name: Check types with PyType
+        run: make pytype-test
+        # Mark this check optional for now, in case it starts out failing.
+        # We will make it required once we've fixed any existing issues.
+        continue-on-error: true
+        # https://github.com/google/pytype/issues/1475
+        #
+        # PyType does not yet support Python 3.12; if this step is enabled, it
+        # fails with an error: "Python versions > 3.11 are not yet supported."
+        if: ${{ matrix.python != '3.12' }}

Makefile

@@ -35,5 +35,11 @@ autopep8:
 	$(VERB) find . -name \*\.py \
 	        | xargs -I {} python $(THIRD_PARTY_PYTHON)/autopep8.py --in-place {}
 
+mypy-test:
+	$(VERB) python -m mypy --ignore-missing-imports `find . -name 'third_party' -prune -o -name '*.py' -print`
+
+pytype-test:
+	$(VERB) python -m pytype -k `find . -name 'third_party' -prune -o -name '*.py' -print`
+
 clean:
 	$(VERB) rm -rf `find . -name \*\.pyc` $(THIRD_PARTY_PYTHON)/*

requirements-test.txt

@@ -0,0 +1,3 @@
+# Type checking.
+mypy ~= 1.9.0
+pytype ~= 2024.3.19

requirements.txt

@@ -0,0 +1,8 @@
+# We don't need any content here, but the absence of this file breaks
+# actions/setup-python in GitHub Actions, e.g.,
+#
+#   https://github.com/mbrukman/csv2txf/actions/runs/10437183622/job/28903088310?pr=29
+#
+#   > Error: No file in /Users/runner/work/csv2txf/csv2txf matched to
+#   > [**/requirements.txt or **/pyproject.toml], make sure you have checked out
+#   > the target repository