python: make tests valid and more fleshed out

python/ql/test/experimental/dataflow/exceptions/test_group.py

@@ -31,5 +31,47 @@ def test_as_binding():
         e_with_source = Exception()
         e_with_source.a = SOURCE
         raise e_with_source
-    except* Exception as e:
-        SINK(e.a) # $ MISSING: flow
+    except* Exception as eg:
+        SINK(eg.exceptions[0].a) # $ MISSING: flow
+
+@expects(4)
+def test_group():
+    value_error_with_source = ValueError()
+    value_error_with_source.a = SOURCE
+
+    type_error_without_source = TypeError()
+    type_error_without_source.a = NONSOURCE
+
+    os_error_without_source = OSError()
+    os_error_without_source.a = NONSOURCE
+
+    eg = ExceptionGroup(
+        "one",
+        [
+            type_error_without_source,
+            ExceptionGroup(
+                "two",
+                    [type_error_without_source, value_error_with_source]
+            ),
+            ExceptionGroup(
+                    "three",
+                    [os_error_without_source]
+            )
+        ]
+    )
+    try:
+        raise eg
+    except* (TypeError, OSError) as es:
+        # The matched sub-group, represented by `es` is filtered,
+        # but the nesting is in place.
+        SINK_F(es.exceptions[0].a)
+        SINK_F(es.exceptions[1].exceptions[0].a)
+        SINK_F(es.exceptions[2].exceptions[0].a)
+    except* ValueError as es:
+        # The matched sub-group, represented by `es` is filtered,
+        # but the nesting is in place.
+        # So the ValueError was originally found in
+        # `eg.exceptions[1].exceptions[1].a`
+        # but now it is found in
+        # `es.exceptions[0].exceptions[0].a`
+        SINK(es.exceptions[0].exceptions[0].a) # $ MISSING: flow