Start a list of modules which require typing (#8198)

* Start a list of modules which require typing

Notes inline. Just one module so far!

pyproject.toml

@@ -79,7 +79,7 @@ show_error_context = true
 warn_redundant_casts = true
 warn_unused_ignores = true
 
-# Most of the numerical computing stack doesn't have type annotations yet.
+# Much of the numerical computing stack doesn't have type annotations yet.
 [[tool.mypy.overrides]]
 ignore_missing_imports = true
 module = [
@@ -118,6 +118,15 @@ module = [
   "numpy.exceptions.*", # remove once support for `numpy<2.0` has been dropped
 ]
 
+# Gradually we want to add more modules to this list, ratcheting up our total
+# coverage. Once a module is here, functions require annotations in order to
+# pass mypy. It would be especially useful to have tests here, because without
+# annotating test functions, we don't have a great way of testing our type
+# annotations — even with just `-> None` is sufficient for mypy to check them.
+[[tool.mypy.overrides]]
+disallow_untyped_defs = true
+module = ["xarray.core.rolling_exp"]
+
 [tool.ruff]
 builtins = ["ellipsis"]
 exclude = [

xarray/core/rolling_exp.py

@@ -9,18 +9,23 @@
 from xarray.core.options import _get_keep_attrs
 from xarray.core.pdcompat import count_not_none
 from xarray.core.pycompat import is_duck_dask_array
-from xarray.core.types import T_DataWithCoords
+from xarray.core.types import T_DataWithCoords, T_DuckArray
 
 
-def _get_alpha(com=None, span=None, halflife=None, alpha=None):
+def _get_alpha(
+    com: float | None = None,
+    span: float | None = None,
+    halflife: float | None = None,
+    alpha: float | None = None,
+) -> float:
     # pandas defines in terms of com (converting to alpha in the algo)
     # so use its function to get a com and then convert to alpha
 
     com = _get_center_of_mass(com, span, halflife, alpha)
     return 1 / (1 + com)
 
 
-def move_exp_nanmean(array, *, axis, alpha):
+def move_exp_nanmean(array: T_DuckArray, *, axis: int, alpha: float) -> np.ndarray:
     if is_duck_dask_array(array):
         raise TypeError("rolling_exp is not currently support for dask-like arrays")
     import numbagg
@@ -32,15 +37,20 @@ def move_exp_nanmean(array, *, axis, alpha):
         return numbagg.move_exp_nanmean(array, axis=axis, alpha=alpha)
 
 
-def move_exp_nansum(array, *, axis, alpha):
+def move_exp_nansum(array: T_DuckArray, *, axis: int, alpha: float) -> np.ndarray:
     if is_duck_dask_array(array):
         raise TypeError("rolling_exp is not currently supported for dask-like arrays")
     import numbagg
 
     return numbagg.move_exp_nansum(array, axis=axis, alpha=alpha)
 
 
-def _get_center_of_mass(comass, span, halflife, alpha):
+def _get_center_of_mass(
+    comass: float | None,
+    span: float | None,
+    halflife: float | None,
+    alpha: float | None,
+) -> float:
     """
     Vendored from pandas.core.window.common._get_center_of_mass