Copy data.values when ranking

Author: GuessWhoSamFoo

doc/source/whatsnew/v0.22.0.txt

@@ -329,5 +329,7 @@ Categorical
 Other
 ^^^^^
 
-- Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
--
+- Improved error message when attempting to use a Python keyword as an identifier in a numexpr query (:issue:`18221`)
+- Fixed a bug where creating a Series from an array that contains both tz-naive and tz-aware values will result in a Series whose dtype is tz-aware instead of object (:issue:`16406`)
+- Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
+- Bug in :func:`Series.rank` where ``Series`` containing ``NaT`` modifies the ``Series`` inplace (:issue:`18521`)

pandas/core/generic.py

@@ -5937,7 +5937,7 @@ def rank(self, axis=0, method='average', numeric_only=None,
             raise NotImplementedError(msg)
 
         def ranker(data):
-            ranks = algos.rank(data.values, axis=axis, method=method,
+            ranks = algos.rank(data.values.copy(), axis=axis, method=method,
                                ascending=ascending, na_option=na_option,
                                pct=pct)
             ranks = self._constructor(ranks, **data._construct_axes_dict())

pandas/tests/series/test_rank.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-from pandas import compat
+from pandas import compat, Timestamp
 
 import pytest
 
@@ -368,3 +368,11 @@ def test_rank_object_bug(self):
         # smoke tests
         Series([np.nan] * 32).astype(object).rank(ascending=True)
         Series([np.nan] * 32).astype(object).rank(ascending=False)
+
+    def test_rank_modify_inplace(self):
+        # GH 18521
+        df = Series([Timestamp('2017-01-05 10:20:27.569000'), NaT])
+        pre_rank_df = df.copy()
+
+        df.rank()
+        assert_series_equal(df, pre_rank_df)