From 3fd0e2bc19b95e28b29760acbaed3352fd0930a3 Mon Sep 17 00:00:00 2001
From: Kaiqi Dong <kaiqi@kth.se>
Date: Tue, 27 Aug 2019 16:09:41 +0200
Subject: [PATCH] Replace with nested dict raises for overlapping keys (#27696)

---
 doc/source/whatsnew/v1.0.0.rst     |  1 +
 pandas/core/generic.py             |  6 +-----
 pandas/tests/frame/test_replace.py | 18 ++++++++++++------
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index 7fe358d3820f23..7a10447e3ad402 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -207,6 +207,7 @@ ExtensionArray
 Other
 ^^^^^
 - Trying to set the ``display.precision``, ``display.max_rows`` or ``display.max_columns`` using :meth:`set_option` to anything but a ``None`` or a positive int will raise a ``ValueError`` (:issue:`23348`)
+- Using :meth:`DataFrame.replace` with overlapping keys in a nested dictionary will no longer raise, now matching the behavior of a flat dictionary (:issue:`27660`)
 - :meth:`DataFrame.to_csv` and :meth:`Series.to_csv` now support dicts as ``compression`` argument with key ``'method'`` being the compression method and others as additional compression options when the compression method is ``'zip'``. (:issue:`26023`)
 
 
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index fac5e0f085fc62..6ade69fb4ca9d9 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6669,11 +6669,7 @@ def replace(
 
                 for k, v in items:
                     keys, values = list(zip(*v.items())) or ([], [])
-                    if set(keys) & set(values):
-                        raise ValueError(
-                            "Replacement not allowed with "
-                            "overlapping keys and values"
-                        )
+
                     to_rep_dict[k] = list(keys)
                     value_dict[k] = list(values)
 
diff --git a/pandas/tests/frame/test_replace.py b/pandas/tests/frame/test_replace.py
index 2862615ef8585a..b341ed6a52ca57 100644
--- a/pandas/tests/frame/test_replace.py
+++ b/pandas/tests/frame/test_replace.py
@@ -1069,18 +1069,24 @@ def test_replace_truthy(self):
         e = df
         assert_frame_equal(r, e)
 
-    def test_replace_int_to_int_chain(self):
+    def test_nested_dict_overlapping_keys_replace_int(self):
+        # GH 27660 keep behaviour consistent for simple dictionary and
+        # nested dictionary replacement
         df = DataFrame({"a": list(range(1, 5))})
-        with pytest.raises(ValueError, match="Replacement not allowed .+"):
-            df.replace({"a": dict(zip(range(1, 5), range(2, 6)))})
 
-    def test_replace_str_to_str_chain(self):
+        result = df.replace({"a": dict(zip(range(1, 5), range(2, 6)))})
+        expected = df.replace(dict(zip(range(1, 5), range(2, 6))))
+        assert_frame_equal(result, expected)
+
+    def test_nested_dict_overlapping_keys_replace_str(self):
+        # GH 27660
         a = np.arange(1, 5)
         astr = a.astype(str)
         bstr = np.arange(2, 6).astype(str)
         df = DataFrame({"a": astr})
-        with pytest.raises(ValueError, match="Replacement not allowed .+"):
-            df.replace({"a": dict(zip(astr, bstr))})
+        result = df.replace(dict(zip(astr, bstr)))
+        expected = df.replace({"a": dict(zip(astr, bstr))})
+        assert_frame_equal(result, expected)
 
     def test_replace_swapping_bug(self):
         df = pd.DataFrame({"a": [True, False, True]})