Comparison and Ordering

Describing Comparisons and Ordering

  • The class method __bool__() has the following properties:

    • Used for truth-value testing
    • Should return either True or False

  • The class method __hash__() has the following properties:

    • Used for retrieving hash value of an object
    • The hash value is used for truth-value testing
    • The value returned is an integer
    • This integer should be identical for two objects that compare as equal
    • Mutable objects shouldn't override this method
  • The following class uses these special methods:
>>> class Person:
...     def __init__(self, age, name):
...         self.age = age
...         self.name = name
...     def __bool__(self):
...         return self.age > 25
...     def __eq__(self, other):
...         eq_age = self.age == other.age
...         eq_name = self.name == other.name
...         return eq_age and eq_name
...     def __hash__(self):
...         return hash((self.age, self.name))

>>> adam_david = Person(23, 'Adam')
>>> adam_peter = Person(23, 'Adam')
>>> luke_james = Person(23, 'Luke')

>>> bool(adam_david)
False
>>> adam_david == adam_peter
True
>>> adam_david == luke_james
False

>>> hash(23)
23
>>> hash('Adam')
6859281680654920110
>>> hash(adam_david)
-2059381663602380558

Summarizing Special Methods

Method Description
__bool__ Performs truth-value testing
__hash__ Computes an integer hash index

Methods for Comparison

Method Result
__lt__(self, other) self<otherself < other
__le__(self, other) self<=otherself <= other
__gt__(self, other) self>otherself > other
__ge__(self, other) self>=otherself >= other
__eq__(self, other) self==otherself == other
__ne__(self, other) self!=otherself != other

References

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