Python Language Sorting, Minimum and Maximum Make custom classes orderable
Example
min, max, and sorted all need the objects to be orderable. To be properly orderable, the class needs to define all of the 6 methods __lt__, __gt__, __ge__, __le__, __ne__ and __eq__:
class IntegerContainer(object):
def __init__(self, value):
self.value = value
def __repr__(self):
return "{}({})".format(self.__class__.__name__, self.value)
def __lt__(self, other):
print('{!r} - Test less than {!r}'.format(self, other))
return self.value < other.value
def __le__(self, other):
print('{!r} - Test less than or equal to {!r}'.format(self, other))
return self.value <= other.value
def __gt__(self, other):
print('{!r} - Test greater than {!r}'.format(self, other))
return self.value > other.value
def __ge__(self, other):
print('{!r} - Test greater than or equal to {!r}'.format(self, other))
return self.value >= other.value
def __eq__(self, other):
print('{!r} - Test equal to {!r}'.format(self, other))
return self.value == other.value
def __ne__(self, other):
print('{!r} - Test not equal to {!r}'.format(self, other))
return self.value != other.value
Though implementing all these methods would seem unnecessary, omitting some of them will make your code prone to bugs.
Examples:
alist = [IntegerContainer(5), IntegerContainer(3),
IntegerContainer(10), IntegerContainer(7)
]
res = max(alist)
# Out: IntegerContainer(3) - Test greater than IntegerContainer(5)
# IntegerContainer(10) - Test greater than IntegerContainer(5)
# IntegerContainer(7) - Test greater than IntegerContainer(10)
print(res)
# Out: IntegerContainer(10)
res = min(alist)
# Out: IntegerContainer(3) - Test less than IntegerContainer(5)
# IntegerContainer(10) - Test less than IntegerContainer(3)
# IntegerContainer(7) - Test less than IntegerContainer(3)
print(res)
# Out: IntegerContainer(3)
res = sorted(alist)
# Out: IntegerContainer(3) - Test less than IntegerContainer(5)
# IntegerContainer(10) - Test less than IntegerContainer(3)
# IntegerContainer(10) - Test less than IntegerContainer(5)
# IntegerContainer(7) - Test less than IntegerContainer(5)
# IntegerContainer(7) - Test less than IntegerContainer(10)
print(res)
# Out: [IntegerContainer(3), IntegerContainer(5), IntegerContainer(7), IntegerContainer(10)]
sorted with reverse=True also uses __lt__:
res = sorted(alist, reverse=True)
# Out: IntegerContainer(10) - Test less than IntegerContainer(7)
# IntegerContainer(3) - Test less than IntegerContainer(10)
# IntegerContainer(3) - Test less than IntegerContainer(10)
# IntegerContainer(3) - Test less than IntegerContainer(7)
# IntegerContainer(5) - Test less than IntegerContainer(7)
# IntegerContainer(5) - Test less than IntegerContainer(3)
print(res)
# Out: [IntegerContainer(10), IntegerContainer(7), IntegerContainer(5), IntegerContainer(3)]
But sorted can use __gt__ instead if the default is not implemented:
del IntegerContainer.__lt__ # The IntegerContainer no longer implements "less than"
res = min(alist)
# Out: IntegerContainer(5) - Test greater than IntegerContainer(3)
# IntegerContainer(3) - Test greater than IntegerContainer(10)
# IntegerContainer(3) - Test greater than IntegerContainer(7)
print(res)
# Out: IntegerContainer(3)
Sorting methods will raise a TypeError if neither __lt__ nor __gt__ are implemented:
del IntegerContainer.__gt__ # The IntegerContainer no longer implements "greater then"
res = min(alist)
TypeError: unorderable types: IntegerContainer() < IntegerContainer()
functools.total_ordering decorator can be used simplifying the effort of writing these rich comparison methods. If you decorate your class with total_ordering, you need to implement __eq__, __ne__ and only one of the __lt__, __le__, __ge__ or __gt__, and the decorator will fill in the rest:
import functools
@functools.total_ordering
class IntegerContainer(object):
def __init__(self, value):
self.value = value
def __repr__(self):
return "{}({})".format(self.__class__.__name__, self.value)
def __lt__(self, other):
print('{!r} - Test less than {!r}'.format(self, other))
return self.value < other.value
def __eq__(self, other):
print('{!r} - Test equal to {!r}'.format(self, other))
return self.value == other.value
def __ne__(self, other):
print('{!r} - Test not equal to {!r}'.format(self, other))
return self.value != other.value
IntegerContainer(5) > IntegerContainer(6)
# Output: IntegerContainer(5) - Test less than IntegerContainer(6)
# Returns: False
IntegerContainer(6) > IntegerContainer(5)
# Output: IntegerContainer(6) - Test less than IntegerContainer(5)
# Output: IntegerContainer(6) - Test equal to IntegerContainer(5)
# Returns True
Notice how the > (greater than) now ends up calling the less than method, and in some cases even the __eq__ method. This also means that if speed is of great importance, you should implement each rich comparison method yourself.
