Add collections.NamedTuple

1 files changed, 60 insertions, 1 deletions

diff --git a/Lib/collections.py b/Lib/collections.py
index 0d9254c9a7..6e816dc776 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -1,3 +1,62 @@
-__all__ = ['deque', 'defaultdict']
+__all__ = ['deque', 'defaultdict', 'NamedTuple']
 
 from _collections import deque, defaultdict
+from operator import itemgetter as _itemgetter
+import sys as _sys
+
+def NamedTuple(typename, s):
+    """Returns a new subclass of tuple with named fields.
+
+    >>> Point = NamedTuple('Point', 'x y')
+    >>> Point.__doc__           # docstring for the new class
+    'Point(x, y)'
+    >>> p = Point(11, y=22)     # instantiate with positional args or keywords
+    >>> p[0] + p[1]             # works just like the tuple (11, 22)
+    33
+    >>> x, y = p                # unpacks just like a tuple
+    >>> x, y
+    (11, 22)
+    >>> p.x + p.y               # fields also accessable by name
+    33
+    >>> p                       # readable __repr__ with name=value style
+    Point(x=11, y=22)
+
+    """
+
+    field_names = s.split()
+    nargs = len(field_names)
+
+    def __new__(cls, *args, **kwds):
+        if kwds:
+            try:
+                args += tuple(kwds[name] for name in field_names[len(args):])
+            except KeyError, name:
+                raise TypeError('%s missing required argument: %s' % (typename, name))
+        if len(args) != nargs:
+            raise TypeError('%s takes exactly %d arguments (%d given)' % (typename, nargs, len(args)))
+        return tuple.__new__(cls, args)
+
+    repr_template = '%s(%s)' % (typename, ', '.join('%s=%%r' % name for name in field_names))
+
+    m = dict(vars(tuple))       # pre-lookup superclass methods (for faster lookup)
+    m.update(__doc__= '%s(%s)' % (typename, ', '.join(field_names)),
+             __slots__ = (),    # no per-instance dict (so instances are same size as tuples)
+             __new__ = __new__,
+             __repr__ = lambda self, _format=repr_template.__mod__: _format(self),
+             __module__ = _sys._getframe(1).f_globals['__name__'],
+             )
+    m.update((name, property(_itemgetter(index))) for index, name in enumerate(field_names))
+
+    return type(typename, (tuple,), m)
+
+
+if __name__ == '__main__':
+    # verify that instances are pickable
+    from cPickle import loads, dumps
+    Point = NamedTuple('Point', 'x y')
+    p = Point(x=10, y=20)
+    assert p == loads(dumps(p))
+
+    import doctest
+    TestResults = NamedTuple('TestResults', 'failed attempted')
+    print TestResults(*doctest.testmod())