Hash in Python

Python provides in built-in function hash() to generate a hash code. As for your own class, you should implement the __hash__() method. An easy, correct way to implement it is to use a key tuple.

class Book:
    def __init__(self, name, price):
        self._name = name
        self._price = price

    def __hash__(self):
        return hash((self._name, self._price))

Like Java, an important rule to obey is that if a class defines equivalence through __eq__, then any implementation of __hash__ must be consistent, in that if x == y, then hash(x) == hash(y). Note that since Python uses a variable number of bits to store integers, it does not make sense to use bit shift operations on the result.

A good hash function

In summary, we have three primary requirements in implementing a good hash function for a given data type:

• It should be consistent: equal keys must produce the same hash value
• It should be efficient to compute
• It should uniformly distribute the keys

Satisfying these requirements simultaneously is a job for experts. Python Programmers can assume hash() does the job.