Merge pull request #4414 from asottile/starred_with_side_effect

Fix assertion rewriting involving Starred + side-effects

changelog/4412.bugfix.rst

@@ -0,0 +1 @@
+Fix assertion rewriting involving ``Starred`` + side-effects.

src/_pytest/assertion/rewrite.py

@@ -946,7 +946,8 @@ class AssertionRewriter(ast.NodeVisitor):
     def visit_Starred(self, starred):
         # From Python 3.5, a Starred node can appear in a function call
         res, expl = self.visit(starred.value)
-        return starred, "*" + expl
+        new_starred = ast.Starred(res, starred.ctx)
+        return new_starred, "*" + expl
 
     def visit_Call_legacy(self, call):
         """

testing/test_assertrewrite.py

@@ -413,6 +413,19 @@ class TestAssertionRewrite(object):
         )
         testdir.runpytest().assert_outcomes(passed=1)
 
+    @pytest.mark.skipif("sys.version_info < (3,5)")
+    def test_starred_with_side_effect(self, testdir):
+        """See #4412"""
+        testdir.makepyfile(
+            """\
+            def test():
+                f = lambda x: x
+                x = iter([1, 2, 3])
+                assert 2 * next(x) == f(*[next(x)])
+            """
+        )
+        testdir.runpytest().assert_outcomes(passed=1)
+
     def test_call(self):
         def g(a=42, *args, **kwargs):
             return False