Python NotImplemented constant

NotImplemented allows you to indicate that a comparison between the two given operands has not been implemented (rather than indicating that the comparison is valid, but yields False, for the two operands).

From the Python Language Reference:

For objects x and y, first x.__op__(y) is tried. If this is not implemented or returns NotImplemented, y.__rop__(x) is tried. If this is also not implemented or returns NotImplemented, a TypeError exception is raised. But see the following exception:

Exception to the previous item: if the left operand is an instance of a built-in type or a new-style class, and the right operand is an instance of a proper subclass of that type or class and overrides the base's __rop__() method, the right operand's __rop__() method is tried before the left operand's __op__() method. This is done so that a subclass can completely override binary operators. Otherwise, the left operand's __op__() method would always accept the right operand: when an instance of a given class is expected, an instance of a subclass of that class is always acceptable.

It actually has the same meaning when returned from __add__ as from __lt__, the difference is Python 2.x is trying other ways of comparing the objects before giving up. Python 3.x does raise a TypeError. In fact, Python can try other things for __add__ as well, look at __radd__ and (though I'm fuzzy on it) __coerce__.

# 2.6
>>> class A(object):
...   def __lt__(self, other):
...     return NotImplemented
>>> A() < A()
True

# 3.1
>>> class A(object):
...   def __lt__(self, other):
...     return NotImplemented
>>> A() < A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: A() < A()

See Ordering Comparisions (3.0 docs) for more info.