fix unnecessary sort in pd.read_json and orient="index"

doc/source/whatsnew/v1.0.0.rst

@@ -311,6 +311,7 @@ I/O
 - Bug in :func:`DataFrame.to_string` where values were truncated using display options instead of outputting the full content (:issue:`9784`)
 - Bug in :meth:`DataFrame.to_json` where a datetime column label would not be written out in ISO format with ``orient="table"`` (:issue:`28130`)
 - Bug in :func:`DataFrame.to_parquet` where writing to GCS would fail with `engine='fastparquet'` if the file did not already exist (:issue:`28326`)
+- Bug in :meth:`DataFrame.read_json` where using ``orient="index"`` would not maintain the order (:issue:`28557`)
 - Bug in :meth:`DataFrame.to_html` where the length of the ``formatters`` argument was not verified (:issue:`28469`)
 
 Plotting

pandas/io/json/_json.py

@@ -12,7 +12,7 @@
 
 from pandas.core.dtypes.common import ensure_str, is_period_dtype
 
-from pandas import DataFrame, MultiIndex, Series, isna, to_datetime
+from pandas import DataFrame, MultiIndex, Series, compat, isna, to_datetime
 from pandas._typing import Scalar
 from pandas.core.reshape.concat import concat
 
@@ -1112,15 +1112,13 @@ def _parse_no_numpy(self):
             self.check_keys_split(decoded)
             self.obj = DataFrame(dtype=None, **decoded)
         elif orient == "index":
-            self.obj = (
-                DataFrame.from_dict(
-                    loads(json, precise_float=self.precise_float),
-                    dtype=None,
-                    orient="index",
-                )
-                .sort_index(axis="columns")
-                .sort_index(axis="index")
+            self.obj = DataFrame.from_dict(
+                loads(json, precise_float=self.precise_float),
+                dtype=None,
+                orient="index",
             )
+            if compat.PY35:
+                self.obj = self.obj.sort_index(axis="columns").sort_index(axis="index")
         elif orient == "table":
             self.obj = parse_table_schema(json, precise_float=self.precise_float)
         else:

pandas/tests/io/json/test_pandas.py

@@ -166,8 +166,7 @@ def test_roundtrip_simple(self, orient, convert_axes, numpy, dtype):
 
         expected = self.frame.copy()
 
-        if not numpy and (orient == "index" or (PY35 and orient == "columns")):
-            # TODO: debug why sort is required
+        if not numpy and PY35 and orient in ("index", "columns"):
             expected = expected.sort_index()
 
         assert_json_roundtrip_equal(result, expected, orient)
@@ -181,7 +180,7 @@ def test_roundtrip_intframe(self, orient, convert_axes, numpy, dtype):
             data, orient=orient, convert_axes=convert_axes, numpy=numpy, dtype=dtype
         )
         expected = self.intframe.copy()
-        if not numpy and (orient == "index" or (PY35 and orient == "columns")):
+        if not numpy and PY35 and orient in ("index", "columns"):
             expected = expected.sort_index()
 
         if (
@@ -216,7 +215,7 @@ def test_roundtrip_str_axes(self, orient, convert_axes, numpy, dtype):
         )
 
         expected = df.copy()
-        if not numpy and (orient == "index" or (PY35 and orient == "columns")):
+        if not numpy and PY35 and orient in ("index", "columns"):
             expected = expected.sort_index()
 
         if not dtype: