Python NotImplemented constant

Question

Looking through decimal.py, it uses NotImplemented in many special methods. e.g.

class A(object):
    def __lt__(self, a):
                 


        

Answer 1

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 "", line 1, in 
TypeError: unorderable types: A() < A()

See Ordering Comparisions (3.0 docs) for more info.