The ins and outs of overriding comparison operators in Python, from a talk I gave at PyGotham 2014.

1. Getting Rich With Comparison Methods
Matt Story (@mscottstory) // PyGotham – August, 2014

2. eng.handshake.com
@handshakeeng

3. $2.50

4. The most certain of all basic
principles is that contradictory
propositions are not true
simultaneously.
~ Aristotle -- Metaphisics

5. >>> from philosopher import Philosopher
>>> aristotle = Philosopher.load(“Aristotle”)
>>> aristotle == Philosopher.load(“Aristotle”)
True

6. >>> from philosopher import Philosopher
>>> aristotle = Philosopher.load(“Aristotle”)
>>> aristotle == Philosopher.load(“Aristotle”)
True
>>> aristotle != Philosopher.load(“Aristotle”)
True

7. How Am I Not Myself?

8. class Philosopher(JSONStore):
'''JSONStore provides `load` and `save`'''
store = './lyceum'
dump_these = ('name', ‘rep’, 'works’)
def __init__(self, name, rep=5, works=tuple()):
self.name = name
self.rep = rep
self.works = list(works)
super(Philosopher,
self).__init__(name.lower())
def __eq__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, ‘id’):
return self.id == other.id
return NotImplemented

9. class Philosopher(JSONStore):
'''JSONStore provides `load` and `save`'''
store = './lyceum'
dump_these = ('name', ‘rep’, 'works’)
def __init__(self, name, rep=5, works=tuple()):
self.name = name
self.rep = rep
self.works = list(works)
super(Philosopher,
self).__init__(name.lower())
def __eq__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, ‘id’):
return self.id == other.id
return NotImplemented

11. There are no implied relationships among the
comparison operators. The truth of x == y does
not imply that x != y is false. Accordingly, when
defining __eq__(), one should also define
__ne__() so that the operators will behave as
expected.
5 Minutes Later …

12. class Philosopher(JSONStore):
# ... snip
def __eq__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, ‘id’):
return self.id == other.id
return NotImplemented
def __ne__(self, other):
is_eq = self.__eq__(other)
if is_eq is NotImplemented:
return is_eq
return not is_eq

13. class Philosopher(JSONStore):
# ... snip
def __eq__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, ‘id’):
return self.id == other.id
return NotImplemented
def __ne__(self, other):
is_eq = self.__eq__(other)
if is_eq is NotImplemented:
return is_eq
return not is_eq

14. >>> aristotle == Philosopher.load(“Aristotle”)
True
>>> aristotle != Philosopher.load(“Aristotle”)
False

15. Aristotle
KANT

16. class Philosopher(JSONStore):
# ... Snip
def __gt__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, 'rep'):
return self.rep > other.rep
return NotImplemented

17. >>> aristotle = Philosopher.load(‘Aristotle’)
>>> kant = Philosopher.load(‘Kant’)
>>> print aristotle.rep
10
>>> print kant.rep
8
>>> aristotle > kant
True

18. >>> aristotle = Philosopher.load(‘Aristotle’)
>>> kant = Philosopher.load(‘Kant’)
>>> print aristotle.rep
10
>>> print kant.rep
8
>>> aristotle > kant
True
>>> kant < aristotle
True

20. 5 Seconds Later …
There are no swapped-argument versions of
these methods (to be used when the left
argument does not support the operation but
the right argument does); rather, __lt__() and
__gt__() are each other’s reflection, __le__()
and __ge__() are each other’s reflection, and
__eq__() and __ne__() are their own reflection.

21. class Philosopher(JSONStore):
# ... Snip
def __gt__(self, other):
print ’{}: called __gt__’.format(self.name)
if isinstance(self, other.__class__)
and hasattr(other, 'rep'):
return self.rep > other.rep
return NotImplemented

22. >>> aristotle > kant
Aristotle: called __gt__
True
>>> kant < aristotle
Aristotle: called __gt__
True

23. NotImplemented
This type has a single value. There is a single
object with this value … Rich comparison
methods may return this value if they do not
implement the operation for the operands
provided. (The interpreter will then try the
reflected operation, or some other fallback,
depending on the operator.)

24. >>> "abc".__lt__(0)
NotImplemented

25. Right-Side
Reflection Returns
NotImplemented
Left-Side Method
Is Defined
Yes
No
Left-Side Method
Returns
NotImplemented
Yes
No
Return
Right-Side
Reflection Is
Defined
Return
Yes
No
Return Default (is)
No

26. Aristotle
Foucault

27. class PostModernist(Philosopher):
def __lt__(self, other):
if isinstance(self, other.__class__):
# with panopticon authority
return False
return NotImplemented

28. >>> from philosopher import PostModernist
>>> foucault = PostModernist.load('Foucault')
>>> foucault.rep
6
>>> aristotle.rep
10
>>> aristotle > foucault

29. >>> from philosopher import PostModernist
>>> foucault = PostModernist.load('Foucault')
>>> foucault.rep
6
>>> aristotle.rep
10
>>> aristotle > foucault
False

30. class PostModernist(Philosopher):
def __lt__(self, other):
print ’{}: __lt__ called'.format(self.name)
if isinstance(self, other.__class__):
return False
return NotImplemented

31. >>> aristotle > foucault
Foucault: called __lt__
False

33. import operator
assert operator.lt(kant, aristotle)
assert operator.gt(aristotle, kant)
assert operator.eq(kant, kant)
assert operator.eq(aristotle, kant)

34. __eq__
__ne__
__lt__
__le__
__gt__
__ge__
Lots of methods, means lots of redundant code

35. @functools.total_ordering
class Philosopher(JSONStore):
# ... snip
def __eq__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, ‘id’):
return self.id == other.id
return NotImplemented
def __gt__(self, other):
if isinstance(self, other.__class__)
and hasattr(other, 'rep'):
return self.rep > other.rep
return NotImplemented
def __ne__(self, other):
# snip ...

37. class X(object):
def __eq__(self, other):
return lambda x: x == other
def __ne__(self, other):
return lambda x: x != other
def __lt__(self, other):
return lambda x: x < other
def __le__(self, other):
return lambda x: x <= other
def __gt__(self, other):
return lambda x: x > other
def __ge__(self, other):
return lambda x: x >= other
x = X()

38. >>> filter(x == 2, range(5))
[2]
>>> filter(x > 1, range(5))
[2, 3, 4]

40. Thanks
@mscottstory