FIX: encoding typing

xarray/backends/netCDF4_.py

@@ -142,7 +142,7 @@ def _check_encoding_dtype_is_vlen_string(dtype):
 
 def _get_datatype(
     var, nc_format="NETCDF4", raise_on_invalid_encoding=False
-) -> np.dtype | None:
+) -> np.dtype:
     if nc_format == "NETCDF4":
         return _nc4_dtype(var)
     if "dtype" in var.encoding:
@@ -421,7 +421,7 @@ def open_store_variable(self, name: str, var):
         dimensions = var.dimensions
         attributes = {k: var.getncattr(k) for k in var.ncattrs()}
         data = indexing.LazilyIndexedArray(NetCDF4ArrayWrapper(name, self))
-        encoding = {}
+        encoding: dict[str, Any] = {}
         if isinstance(var.datatype, netCDF4.EnumType):
             encoding["dtype"] = np.dtype(
                 data.dtype,
@@ -507,11 +507,11 @@ def prepare_variable(
         )
         # check enum metadata and use netCDF4.EnumType
         if (
-            np.dtype(datatype).metadata
-            and datatype.metadata.get("enum_name")
-            and datatype.metadata.get("enum")
+            (meta := np.dtype(datatype).metadata)
+            and (e_name := meta.get("enum_name"))
+            and (e_dict := meta.get("enum"))
         ):
-            datatype = self._build_and_get_enum(name, datatype)
+            datatype = self._build_and_get_enum(name, datatype, e_name, e_dict)
         encoding = _extract_nc4_variable_encoding(
             variable, raise_on_invalid=check_encoding, unlimited_dims=unlimited_dims
         )
@@ -542,10 +542,14 @@ def prepare_variable(
 
         return target, variable.data
 
-    def _build_and_get_enum(self, var_name: str, dtype: np.dtype) -> object:
-        """Add or get the netCDF4 Enum based on the dtype in encoding."""
-        enum_dict = dtype.metadata["enum"]
-        enum_name = dtype.metadata["enum_name"]
+    def _build_and_get_enum(
+        self, var_name: str, dtype: np.dtype, enum_name: str, enum_dict: dict[str, int]
+    ) -> Any:
+        """
+        Add or get the netCDF4 Enum based on the dtype in encoding.
+        The return type should be ``netCDF4.EnumType``,
+        but we avoid importing netCDF4 globally for performances.
+        """
         if enum_name not in self.ds.enumtypes:
             return self.ds.createEnumType(
                 dtype,