typing annotations

.pre-commit-config.yaml

@@ -28,3 +28,21 @@ repos:
       rev: v5.6.4
       hooks:
       -   id: isort
+
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.910-1
+    hooks:
+      - id: mypy
+        #  `asv_bench` are copied from setup.cfg.
+        # `_typed_ops.py` is added since otherwise mypy will complain (but notably only in pre-commit)
+        exclude: "asv_bench"
+        additional_dependencies: [
+            # Type stubs
+            types-python-dateutil,
+            types-pkg_resources,
+            types-PyYAML,
+            types-pytz,
+            typing-extensions==3.10.0.0,
+            # Dependencies that are typed
+            numpy,
+          ]

CHANGELOG.rst

@@ -45,12 +45,10 @@ New Features
   :py:meth:`~climpred.classes.PerfectModelEnsemble.bootstrap` to group skill by
   initializations seasonality. (:issue:`635`, :pr:`690`) `Aaron Spring`_.
 
-Documentation
--------------
-
 Internals/Minor Fixes
 ---------------------
 - Reduce dependencies (:pr:`686`) `Aaron Spring`_.
+- Add `typing <https://docs.python.org/3/library/typing.html>`_ (:issue:`685`, :pr:`692`) `Aaron Spring`_.
 
 
 climpred v2.1.6 (2021-08-31)

ci/requirements/climpred-dev.yml

@@ -30,7 +30,7 @@ dependencies:
   - black==19.10b0
   - coveralls
   - flake8
-  #- importlib_metadata
+  - mypy
   - isort
   - pre-commit
   - pylint

climpred/checks.py

@@ -307,7 +307,10 @@ def warn_if_chunking_would_increase_performance(ds, crit_size_in_MB=100):
             )
 
 
-def _check_valid_reference(reference):
+from typing import List, Optional, Union
+
+
+def _check_valid_reference(reference: Optional[Union[List[str], str]]) -> List:
     """Enforce reference as list and check for valid entries."""
     if reference is None:
         reference = []