Update infer_dtype_from

Author: kotamatsuoka

pandas/core/dtypes/cast.py

@@ -684,7 +684,9 @@ def infer_dtype_from_scalar(val, pandas_dtype: bool = False):
         # instead of np.empty (but then you still don't want things
         # coming out as np.str_!
 
-        dtype = np.object_
+        from pandas.core.arrays.string_ import StringDtype
+
+        dtype = StringDtype()
 
     elif isinstance(val, (np.datetime64, datetime)):
         val = tslibs.Timestamp(val)

pandas/core/internals/blocks.py

@@ -1006,14 +1006,8 @@ def coerce_to_target_dtype(self, other):
         we can also safely try to coerce to the same dtype
         and will receive the same block
         """
-        if isinstance(other, str):
-            if is_string_dtype(self.dtype):
-                return self
-            else:
-                dtype = np.object_
-        else:
-            # if we cannot then coerce to object
-            dtype, _ = infer_dtype_from(other, pandas_dtype=True)
+        # if we cannot then coerce to object
+        dtype, _ = infer_dtype_from(other, pandas_dtype=True)
 
         if is_dtype_equal(self.dtype, dtype):
             return self

pandas/tests/series/methods/test_replace.py

@@ -278,7 +278,7 @@ def test_replace_replacer_equals_replacement(self):
         # GH 20656
         # make sure all replacers are matching against original values
         s = pd.Series(["a", "b"])
-        expected = pd.Series(["b", "a"])
+        expected = pd.Series(["b", "a"], dtype="string")
         result = s.replace({"a": "b", "b": "a"})
         tm.assert_series_equal(expected, result)
 
@@ -350,19 +350,24 @@ def test_replace_with_no_overflowerror(self):
         tm.assert_series_equal(result, expected)
 
     @pytest.mark.parametrize(
-        "ser, to_replace, exp",
+        "ser, to_replace, exp, dtype",
         [
-            ([1, 2, 3], {1: 2, 2: 3, 3: 4}, [2, 3, 4]),
-            (["1", "2", "3"], {"1": "2", "2": "3", "3": "4"}, ["2", "3", "4"]),
+            ([1, 2, 3], {1: 2, 2: 3, 3: 4}, [2, 3, 4], "int64"),
+            (
+                ["1", "2", "3"],
+                {"1": "2", "2": "3", "3": "4"},
+                ["2", "3", "4"],
+                "string",
+            ),
         ],
     )
-    def test_replace_commutative(self, ser, to_replace, exp):
+    def test_replace_commutative(self, ser, to_replace, exp, dtype):
         # GH 16051
         # DataFrame.replace() overwrites when values are non-numeric
 
         series = pd.Series(ser)
 
-        expected = pd.Series(exp)
+        expected = pd.Series(exp, dtype=dtype)
         result = series.replace(to_replace)
 
         tm.assert_series_equal(result, expected)