Users demand iterable input for named tuples. The author capitulates.

Doc/library/collections.rst

@@ -391,6 +391,8 @@ Example::
            def __new__(cls, x, y):
                return tuple.__new__(cls, (x, y))
 
+           _cast = classmethod(tuple.__new__)
+
            def __repr__(self):
                return 'Point(x=%r, y=%r)' % self
 
@@ -400,7 +402,7 @@ Example::
 
            def _replace(self, **kwds):
                'Return a new Point object replacing specified fields with new values'
-               return Point(*map(kwds.get, ('x', 'y'), self))
+               return Point._cast(map(kwds.get, ('x', 'y'), self))
 
            @property
            def _fields(self):
@@ -425,33 +427,30 @@ by the :mod:`csv` or :mod:`sqlite3` modules::
 
    EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade')
 
-   from itertools import starmap
    import csv
-   for emp in starmap(EmployeeRecord, csv.reader(open("employees.csv", "rb"))):
+   for emp in map(EmployeeRecord._cast, csv.reader(open("employees.csv", "rb"))):
        print emp.name, emp.title
 
    import sqlite3
    conn = sqlite3.connect('/companydata')
    cursor = conn.cursor()
    cursor.execute('SELECT name, age, title, department, paygrade FROM employees')
-   for emp in starmap(EmployeeRecord, cursor.fetchall()):
+   for emp in map(EmployeeRecord._cast, cursor.fetchall()):
        print emp.name, emp.title
 
-When casting a single record to a named tuple, use the star-operator [#]_ to unpack
-the values::
+In addition to the methods inherited from tuples, named tuples support
+three additonal methods and a read-only attribute.
 
-   >>> t = [11, 22]
-   >>> Point(*t)               # the star-operator unpacks any iterable object
-   Point(x=11, y=22)
+.. method:: namedtuple._cast(iterable)
 
-When casting a dictionary to a named tuple, use the double-star-operator::
+   Class method returning a new instance taking the positional arguments from the *iterable*.
+   Useful for casting existing sequences and iterables to named tuples:
 
-   >>> d = {'x': 11, 'y': 22}
-   >>> Point(**d)
-   Point(x=11, y=22)
+::
 
-In addition to the methods inherited from tuples, named tuples support
-two additonal methods and a read-only attribute.
+   >>> t = [11, 22]
+   >>> Point._cast(t)
+   Point(x=11, y=22)
 
 .. method:: somenamedtuple._asdict()
 
@@ -498,6 +497,12 @@ function:
     >>> getattr(p, 'x')
     11
 
+When casting a dictionary to a named tuple, use the double-star-operator [#]_::
+
+   >>> d = {'x': 11, 'y': 22}
+   >>> Point(**d)
+   Point(x=11, y=22)
+
 Since a named tuple is a regular Python class, it is easy to add or change
 functionality.  For example, the display format can be changed by overriding
 the :meth:`__repr__` method:
@@ -520,5 +525,5 @@ and customizing it with :meth:`_replace`:
 
 .. rubric:: Footnotes
 
-.. [#] For information on the star-operator see
+.. [#] For information on the double-star-operator see
    :ref:`tut-unpacking-arguments` and :ref:`calls`.

Lib/collections.py

@@ -62,6 +62,7 @@ def namedtuple(typename, field_names, verbose=False):
         __slots__ = () \n
         def __new__(cls, %(argtxt)s):
             return tuple.__new__(cls, (%(argtxt)s)) \n
+        _cast = classmethod(tuple.__new__) \n
         def __repr__(self):
             return '%(typename)s(%(reprtxt)s)' %% self \n
         def _asdict(t):
@@ -69,7 +70,7 @@ 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(*map(kwds.get, %(field_names)r, self)) \n
+            return %(typename)s._cast(map(kwds.get, %(field_names)r, self)) \n
         @property
         def _fields(self):
             return %(field_names)r \n\n''' % locals()

Lib/test/test_collections.py

@@ -46,6 +46,7 @@ class TestNamedTuple(unittest.TestCase):
         self.assertEqual(repr(p), 'Point(x=11, y=22)')
         self.assert_('__dict__' not in dir(p))                              # verify instance has no dict
         self.assert_('__weakref__' not in dir(p))
+        self.assertEqual(p, Point._cast([11, 22]))                          # test _cast classmethod
         self.assertEqual(p._fields, ('x', 'y'))                             # test _fields attribute
         self.assertEqual(p._replace(x=1), (1, 22))                          # test _replace method
         self.assertEqual(p._asdict(), dict(x=11, y=22))                     # test _asdict method
@@ -90,12 +91,14 @@ class TestNamedTuple(unittest.TestCase):
     def test_odd_sizes(self):
         Zero = namedtuple('Zero', '')
         self.assertEqual(Zero(), ())
+        self.assertEqual(Zero._cast([]), ())
         self.assertEqual(repr(Zero()), 'Zero()')
         self.assertEqual(Zero()._asdict(), {})
         self.assertEqual(Zero()._fields, ())
 
         Dot = namedtuple('Dot', 'd')
         self.assertEqual(Dot(1), (1,))
+        self.assertEqual(Dot._cast([1]), (1,))
         self.assertEqual(Dot(1).d, 1)
         self.assertEqual(repr(Dot(1)), 'Dot(d=1)')
         self.assertEqual(Dot(1)._asdict(), {'d':1})
@@ -108,6 +111,7 @@ class TestNamedTuple(unittest.TestCase):
         Big = namedtuple('Big', names)
         b = Big(*range(n))
         self.assertEqual(b, tuple(range(n)))
+        self.assertEqual(Big._cast(range(n)), tuple(range(n)))
         for pos, name in enumerate(names):
             self.assertEqual(getattr(b, name), pos)
         repr(b)                                 # make sure repr() doesn't blow-up