Add tests for multiple assignments and unpacking

tests/syntax/test_assignments.py

@@ -0,0 +1,54 @@
+# Minimal semantics tests for multiple assignment and LHS unpacking.
+
+import pytest
+
+from tests.utils import sampleParamPFrom
+
+
+def test_multiple_assignment_swap():
+    p = sampleParamPFrom(
+        """
+        a = 1
+        b = 2
+        a, b = b, a
+        param p = (a, b)
+        """
+    )
+    assert p == (2, 1)
+
+
+def test_lhs_tuple_unpack():
+    p = sampleParamPFrom(
+        """
+        x, y = (3, 4)
+        param p = (x, y)
+        """
+    )
+    assert p == (3, 4)
+
+
+def test_unpack_from_distribution_pair():
+    code = """
+        pair = (Uniform(0, 1), Uniform(0, 1))
+        x, y = pair
+        param p = (x, y)
+    """
+    outs = [sampleParamPFrom(code) for _ in range(20)]
+    assert len(set(outs)) > 1
+    for x, y in outs:
+        assert 0 <= x <= 1
+        assert 0 <= y <= 1
+
+
+@pytest.mark.parametrize(
+    "rhs",
+    ["(1, 2, 3)", "(1,)"],  # too many; not enough
+)
+def test_unpack_arity_mismatch_exec_raises(rhs):
+    with pytest.raises(ValueError):
+        sampleParamPFrom(
+            f"""
+            x, y = {rhs}
+            param p = x
+            """
+        )

tests/syntax/test_parser.py

@@ -168,6 +168,20 @@ def test_workspace_assign(self):
                 assert False
 
 
+class TestAssign:
+    def test_tuple_lhs_parses(self):
+        mod = parse_string_helper("a, b = 1, 2")
+        stmt = mod.body[0]
+        match stmt:
+            case Assign(
+                targets=[Tuple([Name("a"), Name("b")])],
+                value=Tuple([Constant(1), Constant(2)]),
+            ):
+                assert True
+            case _:
+                assert False
+
+
 class TestInitialScenario:
     def test_initial_scenario(self):
         mod = parse_string_helper("initial scenario")