[python-package] add type hints for functions accepting dtypes (#5773)

python-package/lightgbm/basic.py

@@ -240,7 +240,7 @@ def _is_numpy_column_array(data: Any) -> bool:
     return len(shape) == 2 and shape[1] == 1
 
 
-def _cast_numpy_array_to_dtype(array: np.ndarray, dtype: np.dtype) -> np.ndarray:
+def _cast_numpy_array_to_dtype(array: np.ndarray, dtype: "np.typing.DTypeLike") -> np.ndarray:
     """Cast numpy array to given dtype."""
     if array.dtype == dtype:
         return array
@@ -264,7 +264,7 @@ def _is_1d_collection(data: Any) -> bool:
 
 def _list_to_1d_numpy(
     data: Any,
-    dtype=np.float32,
+    dtype: "np.typing.DTypeLike" = np.float32,
     name: str = 'list'
 ) -> np.ndarray:
     """Convert data to numpy 1-D array."""
@@ -303,7 +303,11 @@ def _is_2d_collection(data: Any) -> bool:
     )
 
 
-def _data_to_2d_numpy(data: Any, dtype: type = np.float32, name: str = 'list') -> np.ndarray:
+def _data_to_2d_numpy(
+    data: Any,
+    dtype: "np.typing.DTypeLike" = np.float32,
+    name: str = 'list'
+) -> np.ndarray:
     """Convert data to numpy 2-D array."""
     if _is_numpy_2d_array(data):
         return _cast_numpy_array_to_dtype(data, dtype)
@@ -612,7 +616,7 @@ def _c_int_array(data):
     return (ptr_data, type_data, data)  # return `data` to avoid the temporary copy is freed
 
 
-def _is_allowed_numpy_dtype(dtype) -> bool:
+def _is_allowed_numpy_dtype(dtype: type) -> bool:
     float128 = getattr(np, 'float128', type(None))
     return (
         issubclass(dtype, (np.integer, np.floating, np.bool_))