diff --git a/sphinx/domains/python.py b/sphinx/domains/python.py
index fd6a788921c..46a67b4ea3a 100644
--- a/sphinx/domains/python.py
+++ b/sphinx/domains/python.py
@@ -495,7 +495,17 @@ def handle_signature(self, sig: str, signode: desc_signature) -> Tuple[str, str]
 
         sig_prefix = self.get_signature_prefix(sig)
         if sig_prefix:
-            signode += addnodes.desc_annotation(str(sig_prefix), '', *sig_prefix)
+            if type(sig_prefix) is str:
+                warnings.warn(
+                    "Python directive method get_signature_prefix()"
+                    " returning a string is deprecated."
+                    " It must now return a list of nodes."
+                    " Return value was '{}'.".format(sig_prefix),
+                    RemovedInSphinx50Warning)
+                signode += addnodes.desc_annotation(sig_prefix, '',  # type: ignore
+                                                    nodes.Text(sig_prefix))  # type: ignore
+            else:
+                signode += addnodes.desc_annotation(str(sig_prefix), '', *sig_prefix)
 
         if prefix:
             signode += addnodes.desc_addname(prefix, prefix)