Don't accept underscore for namedtuple fields (#2092)

Author: elazarg

mypy/semanal.py

@@ -1656,6 +1656,10 @@ def parse_namedtuple_args(self, call: CallExpr,
                 items, types, ok = self.parse_namedtuple_fields_with_types(listexpr.items, call)
         if not types:
             types = [AnyType() for _ in items]
+        underscore = [item for item in items if item.startswith('_')]
+        if underscore:
+            self.fail("namedtuple() Field names cannot start with an underscore: "
+                      + ', '.join(underscore), call)
         return items, types, ok
 
     def parse_namedtuple_fields_with_types(self, nodes: List[Node],

test-data/unit/check-namedtuple.test

@@ -9,6 +9,10 @@ a = x[1]
 a, b, c = x # E: Need more than 2 values to unpack (3 expected)
 x[2] # E: Tuple index out of range
 
+[case testNamedTupleNoUnderscoreFields]
+from collections import namedtuple
+
+X = namedtuple('X', 'x, _y, _z')  # E: namedtuple() Field names cannot start with an underscore: _y, _z
 
 [case testNamedTupleAccessingAttributes]
 from collections import namedtuple