Minor fix-ups to named tuples:

* Make the _replace() method respect subclassing.

* Using property() to make _fields read-only wasn't a good idea.
  It caused len(Point._fields) to fail.

* Add note to _cast() about length checking and alternative with the star-operator.

2 files changed, 3 insertions, 12 deletions

diff --git a/Lib/collections.py b/Lib/collections.py
index c6d0d0f..487b119 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -60,6 +60,7 @@ def namedtuple(typename, field_names, verbose=False):
     template = '''class %(typename)s(tuple):
         '%(typename)s(%(argtxt)s)' \n
         __slots__ = () \n
+        _fields = %(field_names)r \n
         def __new__(cls, %(argtxt)s):
             return tuple.__new__(cls, (%(argtxt)s)) \n
         _cast = classmethod(tuple.__new__) \n
@@ -70,10 +71,7 @@ def namedtuple(typename, field_names, verbose=False):
             return {%(dicttxt)s} \n
         def _replace(self, **kwds):
             'Return a new %(typename)s object replacing specified fields with new values'
-            return %(typename)s._cast(map(kwds.get, %(field_names)r, self)) \n
-        @property
-        def _fields(self):
-            return %(field_names)r \n\n''' % locals()
+            return self.__class__._cast(map(kwds.get, %(field_names)r, self)) \n\n''' % locals()
     for i, name in enumerate(field_names):
         template += '        %s = property(itemgetter(%d))\n' % (name, i)
     if verbose:
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index edffbbe..5e71399 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -17,6 +17,7 @@ class TestNamedTuple(unittest.TestCase):
         self.assertEqual(Point.__slots__, ())
         self.assertEqual(Point.__module__, __name__)
         self.assertEqual(Point.__getitem__, tuple.__getitem__)
+        self.assertEqual(Point._fields, ('x', 'y'))
 
         self.assertRaises(ValueError, namedtuple, 'abc%', 'efg ghi')       # type has non-alpha char
         self.assertRaises(ValueError, namedtuple, 'class', 'efg ghi')      # type has keyword
@@ -51,14 +52,6 @@ class TestNamedTuple(unittest.TestCase):
         self.assertEqual(p._replace(x=1), (1, 22))                          # test _replace method
         self.assertEqual(p._asdict(), dict(x=11, y=22))                     # test _asdict method
 
-        # Verify that _fields is read-only
-        try:
-            p._fields = ('F1' ,'F2')
-        except AttributeError:
-            pass
-        else:
-            self.fail('The _fields attribute needs to be read-only')
-
         # verify that field string can have commas
         Point = namedtuple('Point', 'x, y')
         p = Point(x=11, y=22)