From cf43256a4f10b7b208329418ccbaf1921dd23d1d Mon Sep 17 00:00:00 2001
From: Isaac Virshup <ivirshup@gmail.com>
Date: Sun, 8 Oct 2023 18:23:13 +0200
Subject: [PATCH] Fix error when concatenating object to categorical (#1171)

* Fix error when concatenating object to categorical

* release note
---
 anndata/_core/merge.py            |  2 +-
 anndata/tests/test_concatenate.py | 26 ++++++++++++++++++++++++++
 docs/release-notes/0.10.1.md      |  2 ++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/anndata/_core/merge.py b/anndata/_core/merge.py
index 3bb9970d5..a99eda6ac 100644
--- a/anndata/_core/merge.py
+++ b/anndata/_core/merge.py
@@ -264,7 +264,7 @@ def try_unifying_dtype(
                 dtypes.add(dtype)
                 ordered = ordered | dtype.ordered
             elif not pd.isnull(dtype):
-                return False
+                return None
         if len(dtypes) > 0 and not ordered:
             categories = reduce(
                 lambda x, y: x.union(y),
diff --git a/anndata/tests/test_concatenate.py b/anndata/tests/test_concatenate.py
index ac9f7b0a9..318e78b3f 100644
--- a/anndata/tests/test_concatenate.py
+++ b/anndata/tests/test_concatenate.py
@@ -1219,6 +1219,32 @@ def test_concat_ordered_categoricals_retained():
     assert c.obs["cat_ordered"].cat.ordered
 
 
+def test_concat_categorical_dtype_promotion():
+    """https://github.com/scverse/anndata/issues/1170
+
+    When concatenating categorical with other dtype, defer to pandas.
+    """
+    a = AnnData(
+        np.ones((3, 3)),
+        obs=pd.DataFrame(
+            {"col": pd.Categorical(["a", "a", "b"])},
+            index=[f"cell_{i:02d}" for i in range(3)],
+        ),
+    )
+    b = AnnData(
+        np.ones((3, 3)),
+        obs=pd.DataFrame(
+            {"col": ["c", "c", "c"]},
+            index=[f"cell_{i:02d}" for i in range(3, 6)],
+        ),
+    )
+
+    result = concat([a, b])
+    expected = pd.concat([a.obs, b.obs])
+
+    assert_equal(result.obs, expected)
+
+
 def test_bool_promotion():
     np_bool = AnnData(
         np.ones((5, 1)),
diff --git a/docs/release-notes/0.10.1.md b/docs/release-notes/0.10.1.md
index 1b83f8906..1fc710342 100644
--- a/docs/release-notes/0.10.1.md
+++ b/docs/release-notes/0.10.1.md
@@ -3,6 +3,8 @@
 ```{rubric} Bugfix
 ```
 
+* Fix `ad.concat` erroring when concatenating a categorical and object column {pr}`1171` {user}`ivirshup`
+
 ```{rubric} Documentation
 ```