From 1dfcf1177f91456c29f3d94c8cf383cb94793672 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 6 Jul 2023 12:57:33 -0700
Subject: [PATCH] BUG: ignoring sort in DTA.factorize (#53992)

---
 pandas/core/arrays/datetimelike.py    | 10 +++++++++-
 pandas/tests/arrays/test_datetimes.py | 13 +++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 86849aa41e3e1..40cd59340f942 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -2211,7 +2211,15 @@ def factorize(
                 codes = codes[::-1]
                 uniques = uniques[::-1]
             return codes, uniques
-        # FIXME: shouldn't get here; we are ignoring sort
+
+        if sort:
+            # algorithms.factorize only passes sort=True here when freq is
+            #  not None, so this should not be reached.
+            raise NotImplementedError(
+                f"The 'sort' keyword in {type(self).__name__}.factorize is "
+                "ignored unless arr.freq is not None. To factorize with sort, "
+                "call pd.factorize(obj, sort=True) instead."
+            )
         return super().factorize(use_na_sentinel=use_na_sentinel)
 
     @classmethod
diff --git a/pandas/tests/arrays/test_datetimes.py b/pandas/tests/arrays/test_datetimes.py
index 2acc7bdc0d902..1fe1d4efbefd7 100644
--- a/pandas/tests/arrays/test_datetimes.py
+++ b/pandas/tests/arrays/test_datetimes.py
@@ -745,3 +745,16 @@ def test_iter_zoneinfo_fold(self, tz):
         right2 = dta.astype(object)[2]
         assert str(left) == str(right2)
         assert left.utcoffset() == right2.utcoffset()
+
+
+def test_factorize_sort_without_freq():
+    dta = DatetimeArray._from_sequence([0, 2, 1])
+
+    msg = r"call pd.factorize\(obj, sort=True\) instead"
+    with pytest.raises(NotImplementedError, match=msg):
+        dta.factorize(sort=True)
+
+    # Do TimedeltaArray while we're here
+    tda = dta - dta[0]
+    with pytest.raises(NotImplementedError, match=msg):
+        tda.factorize(sort=True)