Add type hints to maybe_promote in dtypes.py

xarray/core/dtypes.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
 import functools
+from typing import TYPE_CHECKING
 
 import numpy as np
 
 from xarray.core import utils
 
+if TYPE_CHECKING:
+    from xarray.core.types import Scalar
+
 # Use as a sentinel value to indicate a dtype appropriate NA value.
 NA = utils.ReprObject("<NA>")
 
@@ -44,7 +48,7 @@ def __eq__(self, other):
 )
 
 
-def maybe_promote(dtype):
+def maybe_promote(dtype: np.dtype) -> tuple[np.dtype, Scalar]:
     """Simpler equivalent of pandas.core.common._maybe_promote
 
     Parameters
@@ -58,26 +62,30 @@ def maybe_promote(dtype):
     """
     # N.B. these casting rules should match pandas
     if np.issubdtype(dtype, np.floating):
-        fill_value = np.nan
+        dtype_: np.typing.DTypeLike = dtype
+        fill_value: Scalar = np.nan
     elif np.issubdtype(dtype, np.timedelta64):
         # See https://github.com/numpy/numpy/issues/10685
         # np.timedelta64 is a subclass of np.integer
         # Check np.timedelta64 before np.integer
         fill_value = np.timedelta64("NaT")
+        dtype_ = dtype
     elif np.issubdtype(dtype, np.integer):
-        dtype = np.float32 if dtype.itemsize <= 2 else np.float64
+        dtype_ = np.float32 if dtype.itemsize <= 2 else np.float64
         fill_value = np.nan
     elif np.issubdtype(dtype, np.complexfloating):
+        dtype_ = dtype
         fill_value = np.nan + np.nan * 1j
     elif np.issubdtype(dtype, np.datetime64):
+        dtype_ = dtype
         fill_value = np.datetime64("NaT")
     else:
-        dtype = object
+        dtype_ = object
         fill_value = np.nan
 
-    dtype = np.dtype(dtype)
-    fill_value = dtype.type(fill_value)
-    return dtype, fill_value
+    dtype_out = np.dtype(dtype_)
+    fill_value = dtype_out.type(fill_value)
+    return dtype_out, fill_value
 
 
 NAT_TYPES = {np.datetime64("NaT").dtype, np.timedelta64("NaT").dtype}

xarray/core/types.py

@@ -188,6 +188,7 @@ def copy(
 
 DataVars = Mapping[Any, Any]
 
+Scalar = Union[bool, float, complex, str, np.datetime64, np.timedelta64, datetime.date]
 
 ErrorOptions = Literal["raise", "ignore"]
 ErrorOptionsWithWarn = Literal["raise", "warn", "ignore"]