Add runtime type checking in CI tests

This should prevent TYPE_CHECKING misuse.

It might also found issues not found by static type checking,
typically by the current MyPy.

.github/workflows/ci-additional.yaml

@@ -82,6 +82,46 @@ jobs:
           # `xarray/tests/__init__.py`.
           python -m pytest --doctest-modules xarray --ignore xarray/tests -Werror
 
+  beartype:
+    name: beartype
+    runs-on: "ubuntu-latest"
+    needs: detect-ci-trigger
+    defaults:
+      run:
+        shell: bash -l {0}
+    env:
+      CONDA_ENV_FILE: ci/requirements/environment.yml
+      PYTHON_VERSION: "3.12"
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # Fetch all history for all branches and tags.
+
+      - name: set environment variables
+        run: |
+          echo "TODAY=$(date +'%Y-%m-%d')" >> $GITHUB_ENV
+      - name: Setup micromamba
+        uses: mamba-org/setup-micromamba@v2
+        with:
+          micromamba-version: ${{env.MICROMAMBA_VERSION}}
+          environment-file: ${{env.CONDA_ENV_FILE}}
+          environment-name: xarray-tests
+          create-args: >-
+            python=${{env.PYTHON_VERSION}}
+          cache-environment: true
+          cache-environment-key: "${{runner.os}}-${{runner.arch}}-py${{env.PYTHON_VERSION}}-${{env.TODAY}}-${{hashFiles(env.CONDA_ENV_FILE)}}"
+      - name: Install xarray
+        run: |
+          python -m pip install --no-deps -e .
+      - name: Version info
+        run: |
+          python xarray/util/print_versions.py
+
+      - name: Run beartype
+        run: |
+          python -m pytest --beartype-packages=xarray
+
   mypy:
     name: Mypy
     runs-on: "ubuntu-latest"

ci/requirements/environment.yml

@@ -38,6 +38,7 @@ dependencies:
   - pydap
   - pydap-server
   - pytest
+  - pytest-beartype
   - pytest-cov
   - pytest-env
   - pytest-xdist

pyproject.toml

@@ -41,6 +41,7 @@ dev = [
   "mypy",
   "pre-commit",
   "pytest",
+  "pytest-beartype",
   "pytest-cov",
   "pytest-env",
   "pytest-xdist",