From f92a39d194fca1b52627e8652a17cc983c8956c1 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Fri, 10 Jan 2020 17:44:00 -0800
Subject: [PATCH 1/2] BUG: EAs should not be hashable

---
 pandas/core/arrays/base.py             | 3 +++
 pandas/tests/extension/base/methods.py | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 9723343ea7af5..bd5b0ef390804 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -1073,6 +1073,9 @@ def _reduce(self, name, skipna=True, **kwargs):
         """
         raise TypeError(f"cannot perform {name} with type {self.dtype}")
 
+    def __hash__(self):
+        raise TypeError(f"unhashable type: {repr(type(self).__name__)}")
+
 
 class ExtensionOpsMixin:
     """
diff --git a/pandas/tests/extension/base/methods.py b/pandas/tests/extension/base/methods.py
index 1e427c6319cab..6b75176ebd35b 100644
--- a/pandas/tests/extension/base/methods.py
+++ b/pandas/tests/extension/base/methods.py
@@ -261,6 +261,11 @@ def test_shift_fill_value(self, data):
         expected = data.take([2, 3, 0, 0])
         self.assert_extension_array_equal(result, expected)
 
+    def test_not_hashable(self, data):
+        # We are in general mutable, so not hashable
+        with pytest.raises(TypeError, match="unhashable type"):
+            hash(data)
+
     def test_hash_pandas_object_works(self, data, as_frame):
         # https://github.com/pandas-dev/pandas/issues/23066
         data = pd.Series(data)

From 3851d9095d193d6e553c341a070c175da0a69870 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sun, 19 Jan 2020 13:46:05 -0800
Subject: [PATCH 2/2] docstring

---
 pandas/core/arrays/base.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index bd5b0ef390804..c3c91cea43f6b 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -176,6 +176,9 @@ class ExtensionArray:
        types present.
 
     See :ref:`extending.extension.ufunc` for more.
+
+    By default, ExtensionArrays are not hashable.  Immutable subclasses may
+    override this behavior.
     """
 
     # '_typ' is for pandas.core.dtypes.generic.ABCExtensionArray.