added to_variant python test from the GH issue

python/datafusion/tests/test_expr.py

@@ -139,3 +139,31 @@ def test_relational_expr(test_ctx):
     assert df.filter(col("b") != "beta").count() == 2
 
     assert df.filter(col("a") == "beta").count() == 0
+
+
+def test_expr_to_variant():
+    # Taken from https://github.com/apache/datafusion-python/issues/781
+    from datafusion import SessionContext
+    from datafusion.expr import Filter
+
+
+    def traverse_logical_plan(plan):
+        cur_node = plan.to_variant()
+        if isinstance(cur_node, Filter):
+            return cur_node.predicate().to_variant()
+        if hasattr(plan, 'inputs'):
+            for input_plan in plan.inputs():
+                res = traverse_logical_plan(input_plan)
+                if res is not None:
+                    return res
+
+    ctx = SessionContext()
+    data = {'id': [1, 2, 3], 'name': ['Alice', 'Bob', 'Charlie']}
+    ctx.from_pydict(data, name='table1')
+    query = "SELECT * FROM table1 t1 WHERE t1.name IN ('dfa', 'ad', 'dfre', 'vsa')"
+    logical_plan = ctx.sql(query).optimized_logical_plan()
+    variant = traverse_logical_plan(logical_plan)
+    assert variant is not None
+    assert variant.expr().to_variant().qualified_name() == 'table1.name'
+    assert str(variant.list()) == '[Expr(Utf8("dfa")), Expr(Utf8("ad")), Expr(Utf8("dfre")), Expr(Utf8("vsa"))]'
+    assert not variant.negated()