Add Mypy for static type checking

.github/workflows/type_checks.yml

@@ -0,0 +1,53 @@
+# Static type checks
+#
+# This workflow runs static type checks using mypy.
+#
+# It is run on every commit to the main and pull request branches. It is also
+# scheduled to run daily on the main branch.
+#
+name: Static Type Checks
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+  # Schedule daily tests
+  schedule:
+    - cron: '0 0 * * *'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
+jobs:
+  static_check:
+    name: Static Type Check
+    runs-on: ubuntu-latest
+
+    steps:
+      # Checkout current git repository
+      - name: Checkout
+        uses: actions/[email protected]
+
+      # Setup Python
+      - name: Set up Python
+        uses: actions/[email protected]
+        with:
+          python-version: '3.12'
+
+      - name: Install packages
+        run: |
+          # Need to install four groups of packages:
+          # 1. required packages
+          # 2. optional packages
+          # 3. type checker and stub packages
+          # 4. other packages that are used somewhere in PyGMT
+          python -m pip install \
+            numpy pandas xarray netcdf4 packaging \
+            contextily geopandas ipython rioxarray \
+            mypy pandas-stubs \
+            matplotlib pytest
+          python -m pip list
+
+      - name: Static type check
+        run: make typecheck

.gitignore

@@ -17,6 +17,7 @@ MANIFEST
 .coverage
 coverage.xml
 htmlcov/
+.mypy_cache/
 .pytest_cache/
 .ruff_cache/
 results/

Makefile

@@ -18,6 +18,7 @@ help:
 	@echo "  format         run blackdoc, docformatter and ruff to automatically format the code"
 	@echo "  check          run code style and quality checks (blackdoc, docformatter, ruff)"
 	@echo "  codespell      run codespell to check common misspellings"
+	@echo "  typecheck      run mypy for static type check"
 	@echo "  clean          clean up build and generated files"
 	@echo "  distclean      clean up build and generated files, including project metadata files"
 	@echo ""
@@ -73,12 +74,15 @@ check:
 codespell:
 	@codespell
 
+typecheck:
+	mypy ${PROJECT}
+
 clean:
 	find . -name "*.pyc" -exec rm -v {} +
 	find . -name "*~" -exec rm -v {} +
 	find . -type d -name  "__pycache__" -exec rm -rv {} +
 	rm -rvf build dist .eggs MANIFEST .coverage htmlcov coverage.xml
-	rm -rvf .cache .pytest_cache .ruff_cache
+	rm -rvf .cache .mypy_cache .pytest_cache .ruff_cache
 	rm -rvf $(TESTDIR)
 	rm -rvf baseline
 	rm -rvf result_images

environment.yml

@@ -41,3 +41,6 @@ dependencies:
     - sphinx-design
     - sphinx-gallery
     - sphinx_rtd_theme
+    # Dev dependencies (type hints)
+    - mypy
+    - pandas-stubs

pygmt/datasets/load_remote_dataset.py

@@ -1,7 +1,7 @@
 """
 Internal function to load GMT remote datasets.
 """
-from typing import NamedTuple
+from typing import NamedTuple, Union
 
 from pygmt.exceptions import GMTInvalidInput
 from pygmt.helpers import kwargs_to_strings
@@ -58,7 +58,7 @@ class GMTRemoteDataset(NamedTuple):
     title: str
     name: str
     long_name: str
-    units: str
+    units: Union[str, None]
     resolutions: dict[str, Resolution]
     extra_attributes: dict
 

pygmt/figure.py

@@ -514,7 +514,7 @@ def _repr_html_(self):
         html = '<img src="data:image/png;base64,{image}" width="{width}px">'
         return html.format(image=base64_png.decode("utf-8"), width=500)
 
-    from pygmt.src import (
+    from pygmt.src import (  # type: ignore [misc]
         basemap,
         coast,
         colorbar,

pyproject.toml

@@ -86,6 +86,10 @@ make-summary-multi-line = true
 wrap-summaries = 79
 wrap-descriptions = 79
 
+[tool.mypy]
+exclude = ["pygmt/tests/"]
+ignore_missing_imports = true
+
 [tool.ruff]
 line-length = 88  # E501 (line-too-long)
 show-source = true