PERF: reindex default fill_value dtype

pandas/core/internals/managers.py

@@ -821,10 +821,16 @@ def _make_na_block(
 
         if fill_value is None:
             fill_value = np.nan
+            # GH45857 avoid unnecessary upcasting
+            dtype = interleaved_dtype([blk.dtype for blk in self.blocks])
+            if dtype is not None and np.issubdtype(dtype.type, np.floating):
+                fill_value = dtype.type(fill_value)
+
+        dtype, fill_value = infer_dtype_from_scalar(fill_value)
+
         block_shape = list(self.shape)
         block_shape[0] = len(placement)
 
-        dtype, fill_value = infer_dtype_from_scalar(fill_value)
         # error: Argument "dtype" to "empty" has incompatible type "Union[dtype,
         # ExtensionDtype]"; expected "Union[dtype, None, type, _SupportsDtype, str,
         # Tuple[Any, int], Tuple[Any, Union[int, Sequence[int]]], List[Any], _DtypeDict,

pandas/tests/frame/methods/test_reindex.py

@@ -950,6 +950,12 @@ def test_reindex_with_nans(self):
         expected = df.iloc[[1]]
         tm.assert_frame_equal(result, expected)
 
+    def test_reindex_without_upcasting(self):
+        # GH45857
+        df = DataFrame(np.zeros((10, 10), dtype=np.float32))
+        result = df.reindex(columns=np.arange(5, 15))
+        assert result.dtypes.eq(np.float32).all()
+
     def test_reindex_multi(self):
         df = DataFrame(np.random.randn(3, 3))