Question

What is the most natural way to complete the following code?

import functools

@functools.total_ordering
class X:
    def __init__(self, a):
        self._a = a

    def __eq__(self, other):
        if not isinstance(other, X):
            return False
        return self._a == other._a

    def __lt__(self, other):
        if not isinstance(other, X):
            return ...                    // what should go here?
        return self._a < other._a

if __name__ == '__main__':
    s = [2, 'foo', X(2)]
    s.sort()
    print s

Answer 1

You can choose whatever feels natural to you; False means your instances always sort after other types, True and they’ll be sorted before.

Alternatively, you can return NotImplemented (see the __lt__ and other comparison methods documentation) to signal the comparison is not supported:

def __lt__(self, other):
    if not isinstance(other, X):
        return NotImplemented
    return self._a < other._a

Quoting the documentation:

A rich comparison method may return the singleton NotImplemented if it does not implement the operation for a given pair of arguments. By convention, False and True are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an if statement), Python will call bool() on the value to determine if the result is true or false.