Fix parsing trailing comma after for loop index variables

Fixes #543.

Author: JukkaL

mypy/parse.py

@@ -875,6 +875,7 @@ def parse_for_stmt(self) -> ForStmt:
     def parse_for_index_variables(self) -> Node:
         # Parse index variables of a 'for' statement.
         index_items = List[Node]()
+        force_tuple = False
 
         while True:
             v = self.parse_expression(precedence['in'],
@@ -883,8 +884,11 @@ def parse_for_index_variables(self) -> Node:
             if self.current_str() != ',':
                 break
             self.skip()
+            if self.current_str() == 'in':
+                force_tuple = True
+                break
 
-        if len(index_items) == 1:
+        if len(index_items) == 1 and not force_tuple:
             index = index_items[0]
         else:
             index = TupleExpr(index_items)

mypy/test/data/parse.test

@@ -1503,6 +1503,44 @@ MypyFile:1(
     Block:1(
       PassStmt:2())))
 
+[case testForAndTrailingCommaAfterIndexVar]
+for i, in x:
+    pass
+[out]
+MypyFile:1(
+  ForStmt:1(
+    TupleExpr:1(
+      NameExpr(i))
+    NameExpr(x)
+    Block:1(
+      PassStmt:2())))
+
+[case testListComprehensionAndTrailingCommaAfterIndexVar]
+x = [a for b, in c]
+[out]
+MypyFile:1(
+  AssignmentStmt:1(
+    NameExpr(x)
+    ListComprehension:1(
+      GeneratorExpr:1(
+        NameExpr(a)
+        TupleExpr:1(
+          NameExpr(b))
+        NameExpr(c)))))
+
+[case testForAndTrailingCommaAfterIndexVars]
+for i, j, in x:
+    pass
+[out]
+MypyFile:1(
+  ForStmt:1(
+    TupleExpr:1(
+      NameExpr(i)
+      NameExpr(j))
+    NameExpr(x)
+    Block:1(
+      PassStmt:2())))
+
 [case testGeneratorWithCondition]
 x for y in z if 0
 [out]