gh-98886: Fix issues with dataclass fields with special underscore names (#102032)

This commit prefixes `__dataclass` to several things in the locals dict:
- Names like `_dflt_` (which cause trouble, see first test)
- Names like `_type_` (not known to be able to cause trouble)
- `_return_type` (not known to able to cause trouble)
- `_HAS_DEFAULT_FACTORY` (which causes trouble, see second test)

In addition, this removes `MISSING` from the locals dict. As far as I can tell, this wasn't needed even in the initial implementation of dataclasses.py (and tests on that version passed with it removed). This makes me wary :-)

This is basically a continuation of #96151, where fixing this was welcomed in https://github.com/python/cpython/pull/98143#issuecomment-1280306360

1 files changed, 17 insertions, 0 deletions

diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index affd9ce..6888680 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -285,6 +285,23 @@ class TestCase(unittest.TestCase):
         c = C(5)
         self.assertEqual(c.BUILTINS, 5)
 
+    def test_field_with_special_single_underscore_names(self):
+        # gh-98886
+
+        @dataclass
+        class X:
+            x: int = field(default_factory=lambda: 111)
+            _dflt_x: int = field(default_factory=lambda: 222)
+
+        X()
+
+        @dataclass
+        class Y:
+            y: int = field(default_factory=lambda: 111)
+            _HAS_DEFAULT_FACTORY: int = 222
+
+        assert Y(y=222).y == 222
+
     def test_field_named_like_builtin(self):
         # Attribute names can shadow built-in names
         # since code generation is used.