Ensure signature help label contains parameter substrings per LSP spec

pylsp/plugins/signature.py

@@ -37,11 +37,23 @@ def pylsp_signature_help(config, document, position):
 
     s = signatures[0]
 
+    try:
+        function_sig = s.to_string()
+    except Exception:
+        function_sig = ""
+
     docstring = s.docstring()
 
-    # Docstring contains one or more lines of signature, followed by empty line, followed by docstring
-    function_sig_lines = (docstring.split("\n\n") or [""])[0].splitlines()
-    function_sig = " ".join([line.strip() for line in function_sig_lines])
+    if not function_sig:
+        # Docstring contains one or more lines of signature, followed by empty line, followed by docstring
+        function_sig_lines = (docstring.split("\n\n") or [""])[0].splitlines()
+        function_sig = " ".join([line.strip() for line in function_sig_lines])
+
+    # Ensure LSP 3.17 compliance: parameter labels must be substrings of the signature label.
+    if s.params and not all(p.name in function_sig for p in s.params):
+        name = s.name or "callable"
+        param_list = ", ".join(p.name for p in s.params)
+        function_sig = f"{name}({param_list})"
 
     signature_docstring = s.docstring(raw=True)
     if not signature_config.get("include_docstring", True):
@@ -64,6 +76,7 @@ def pylsp_signature_help(config, document, position):
                 ),
             }
             for p in s.params
+            if p.name in function_sig
         ]
 
     # We only return a single signature because Python doesn't allow overloading

test/plugins/test_signature.py

@@ -118,3 +118,59 @@ def test_signature_without_docstring(
     sigs = sig_info["signatures"]
     assert len(sigs) == 1
     assert sigs[0]["documentation"] == {"kind": "markdown", "value": ""}
+
+
+CALLABLE_CLASS_DOC = """class MyCallable:
+    \"\"\"Docstring for MyCallable\"\"\"
+    def __call__(self, a, b=1):
+        \"\"\"Call docstring\"\"\"
+        pass
+
+c = MyCallable()
+c(
+"""
+
+
+def test_callable_instance_signature(workspace) -> None:
+    # Over '( ' in c(
+    sig_position = {"line": 8, "character": 2}
+    doc = Document(DOC_URI, workspace, CALLABLE_CLASS_DOC)
+
+    sig_info = signature.pylsp_signature_help(doc._config, doc, sig_position)
+
+    sigs = sig_info["signatures"]
+    assert len(sigs) == 1
+    # Parameter labels must be substrings of the signature label per LSP 3.17
+    assert sigs[0]["label"] == "MyCallable(a, b=1)"
+    param_labels = [p["label"] for p in sigs[0]["parameters"]]
+    assert param_labels == ["a", "b"]
+    assert all(label in sigs[0]["label"] for label in param_labels)
+
+
+def test_signature_fallback_when_label_missing_params(workspace, monkeypatch) -> None:
+    # Test fallback when s.to_string() does not contain all parameter names
+    sig_position = {"line": 10, "character": 5}
+    doc = Document(DOC_URI, workspace, DOC)
+
+    orig_jedi_script = doc.jedi_script
+
+    class MockScript:
+        def get_signatures(self, **kwargs):
+            signatures = orig_jedi_script().get_signatures(**kwargs)
+            if not signatures:
+                return signatures
+            s = signatures[0]
+            # Mock to_string to return a label missing the parameters (like ufunc())
+            monkeypatch.setattr(s, "to_string", lambda: "mock_func()")
+            return [s]
+
+    monkeypatch.setattr(doc, "jedi_script", lambda: MockScript())
+
+    sig_info = signature.pylsp_signature_help(doc._config, doc, sig_position)
+    sigs = sig_info["signatures"]
+    assert len(sigs) == 1
+    # Fallback should ensure parameter labels are substrings of signature label
+    assert sigs[0]["label"] == "main(param1, param2)"
+    param_labels = [p["label"] for p in sigs[0]["parameters"]]
+    assert param_labels == ["param1", "param2"]
+    assert all(label in sigs[0]["label"] for label in param_labels)