PERF: reindex default fill_value dtype (pandas-dev#57272)

Author: mroeschke

doc/source/whatsnew/v3.0.0.rst

@@ -170,7 +170,7 @@ Numeric
 Conversion
 ^^^^^^^^^^
 - Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)
--
+- Bug in :meth:`Series.reindex` not maintaining ``float32`` type when a ``reindex`` introduces a missing value (:issue:`45857`)
 
 Strings
 ^^^^^^^

pandas/core/internals/managers.py

@@ -1051,8 +1051,12 @@ def _make_na_block(
             nb = NumpyBlock(vals, placement, ndim=2)
             return nb
 
-        if fill_value is None:
+        if fill_value is None or fill_value is np.nan:
             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)
 
         shape = (len(placement), self.shape[1])
 

pandas/tests/frame/methods/test_reindex.py

@@ -1018,6 +1018,12 @@ 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.default_rng(2).standard_normal((3, 3)))