WIP: Thinking of better way than a copy

Author: GuessWhoSamFoo

pandas/core/generic.py

@@ -5620,7 +5620,7 @@ def rank(self, axis=0, method='average', numeric_only=None,
             raise NotImplementedError(msg)
 
         def ranker(data):
-            ranks = algos.rank(data.values.copy(), axis=axis, method=method,
+            ranks = algos.rank(data.values, axis=axis, method=method,
                                ascending=ascending, na_option=na_option,
                                pct=pct)
             ranks = self._constructor(ranks, **data._construct_axes_dict())

pandas/tests/frame/test_analytics.py

@@ -2174,3 +2174,11 @@ def test_series_broadcasting(self):
             df_nan.clip_lower(s, axis=0)
             for op in ['lt', 'le', 'gt', 'ge', 'eq', 'ne']:
                 getattr(df, op)(s_nan, axis=0)
+
+    def test_series_nat_conversion(self):
+        # GH 18521
+        df = DataFrame(np.random.randn(10, 3), dtype='float64')
+        expected = df.copy()
+        df.rank()
+        result = df
+        tm.assert_frame_equal(result, expected)

pandas/tests/series/test_rank.py

@@ -304,8 +304,9 @@ def test_rank_object_bug(self):
 
     def test_rank_modify_inplace(self):
         # GH 18521
-        df = Series([Timestamp('2017-01-05 10:20:27.569000'), NaT])
-        pre_rank_df = df.copy()
+        s = Series([Timestamp('2017-01-05 10:20:27.569000'), NaT])
+        expected = s.copy()
 
-        df.rank()
-        assert_series_equal(df, pre_rank_df)
+        s.rank()
+        result = s
+        assert_series_equal(result, expected)