bpo-39890: Don't mutate the AST when compiling starred assignments (GH-18833)

brandtbucher · web-flow · commit d5aa2e941ccc · 2020-03-08T03:44:18.000Z
diff --git a/Python/compile.c b/Python/compile.c
@@ -3765,7 +3765,6 @@ assignment_helper(struct compiler *c, asdl_seq *elts)
                     "star-unpacking assignment");
             ADDOP_I(c, UNPACK_EX, (i + ((n-i-1) << 8)));
             seen_star = 1;
-            asdl_seq_SET(elts, i, elt->v.Starred.value);
         }
         else if (elt->kind == Starred_kind) {
             return compiler_error(c,
@@ -3775,7 +3774,10 @@ assignment_helper(struct compiler *c, asdl_seq *elts)
     if (!seen_star) {
         ADDOP_I(c, UNPACK_SEQUENCE, n);
     }
-    VISIT_SEQ(c, expr, elts);
+    for (i = 0; i < n; i++) {
+        expr_ty elt = asdl_seq_GET(elts, i);
+        VISIT(c, expr, elt->kind != Starred_kind ? elt : elt->v.Starred.value);
+    }
     return 1;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -3765,7 +3765,6 @@ assignment_helper(struct compiler c, asdl_seq elts)`
`3765`	`3765`	`"star-unpacking assignment");`
`3766`	`3766`	`ADDOP_I(c, UNPACK_EX, (i + ((n-i-1) << 8)));`
`3767`	`3767`	`seen_star = 1;`
`3768`		`- asdl_seq_SET(elts, i, elt->v.Starred.value);`
`3769`	`3768`	`}`
`3770`	`3769`	`else if (elt->kind == Starred_kind) {`
`3771`	`3770`	`return compiler_error(c,`
`@@ -3775,7 +3774,10 @@ assignment_helper(struct compiler c, asdl_seq elts)`
`3775`	`3774`	`if (!seen_star) {`
`3776`	`3775`	`ADDOP_I(c, UNPACK_SEQUENCE, n);`
`3777`	`3776`	`}`
`3778`		`- VISIT_SEQ(c, expr, elts);`
	`3777`	`+ for (i = 0; i < n; i++) {`
	`3778`	`+ expr_ty elt = asdl_seq_GET(elts, i);`
	`3779`	`+ VISIT(c, expr, elt->kind != Starred_kind ? elt : elt->v.Starred.value);`
	`3780`	`+ }`
`3779`	`3781`	`return 1;`
`3780`	`3782`	`}`
`3781`	`3783`