groupby is converted to a list before adding as dendrogram key

scanpy/plotting/_anndata.py

@@ -2112,7 +2112,10 @@ def _get_dendrogram_key(adata, dendrogram_key, groupby):
     # the `dendrogram_key` can be a bool an NoneType or the name of the
     # dendrogram key. By default the name of the dendrogram key is 'dendrogram'
     if not isinstance(dendrogram_key, str):
-        dendrogram_key = f'dendrogram_{groupby}'
+        if isinstance(groupby, str):
+            dendrogram_key = f'dendrogram_{groupby}'
+        elif isinstance(groupby, list):
+            dendrogram_key = f'dendrogram_{"_".join(groupby)}'
 
     if dendrogram_key not in adata.uns:
         from ..tools._dendrogram import dendrogram

scanpy/tests/test_dendrogram_key_added.py

@@ -0,0 +1,25 @@
+import scanpy as sc
+import numpy as np
+import pytest
+
+n_neighbors = 5
+key = 'test'
+
+
+@pytest.fixture
+def adata():
+    return sc.AnnData(sc.datasets.pbmc68k_reduced())
+
+
+@pytest.mark.parametrize('groupby', ['bulk_labels', ['bulk_labels', 'phase']])
+@pytest.mark.parametrize('key_added', [None, "custom_key"])
+def test_dendrogram_key_added(adata, groupby, key_added):
+    sc.tl.dendrogram(adata, groupby=groupby, key_added=key_added, use_rep="X_pca")
+    if isinstance(groupby, list):
+        dendrogram_key = f'dendrogram_{"_".join(groupby)}'
+    else:
+        dendrogram_key = f'dendrogram_{groupby}'
+
+    if key_added is None:
+        key_added = dendrogram_key
+    assert key_added in adata.uns

scanpy/tools/_dendrogram.py

@@ -156,7 +156,7 @@ def dendrogram(
 
     if inplace:
         if key_added is None:
-            key_added = f'dendrogram_{groupby}'
+            key_added = f'dendrogram_{"_".join(groupby)}'
         logg.info(f'Storing dendrogram info using `.uns[{key_added!r}]`')
         adata.uns[key_added] = dat
     else: