96 – Common __hash__ implementation

The dunder method can be implemented to make your instances hashable. This will let you use your instances of your custom class in dictionaries, sets, caches, and other situations.

The most common approach to implement a reasonable __hash__ is to create a tuple with all the “important attributes” of your instance and then using the hash of that tuple.

As a rule of thumb, these “important attributes” tend to be the ones you would use to determine if two instaces of your class are equal. For example, consider the mockup class Fraction:

class Fraction:
    def __init__(self, numerator, denominator):
        self._num = numerator
        self._den = denominator
        self._is_simplified = ...

The attribute _is_simplified will flag whether the fraction has been simplified to its lowest terms, like in Fraction(1, 3), or not, like in Fraction(3, 9). (Note that 1/3 and 3/9 are the same number.)

The fractions Fraction(1, 3) and Fraction(3, 9) are equal, although their attribute _is_simplified is different, since you can use the attributes _num and _den to determine they have the same numerical value.

Hence, for the class Fraction, you would use the attributes _num and _den to implement __hash__:

class Fraction:
    # ...

    def __hash__(self):
        return hash((self._num, self._den))

Note that, if two instances are considered equal, then their hashes must be the same.