Python hash()

The hash() method returns the hash value of an object if it has one.

Hash values are just integers which are used to compare dictionary keys during a dictionary lookup quickly.

Internally, hash() method calls __hash__() method of an object which are set by default for any object. We'll look at this later.

The syntax of hash() method is:

hash(object)

hash() Parameters

The hash() method takes a single parameter:

  • object - the object whose hash value is to be returned (integer, string, float)

Return value from hash()

The hash() method returns the hash value of an object if it has one.

If an object has custom __hash__() method, it truncates the return value to the size of Py_ssize_t.

Example 1: How hash() works in Python?

# hash for integer unchanged
print('Hash for 181 is:', hash(181))

# hash for decimal
print('Hash for 181.23 is:',hash(181.23))

# hash for string
print('Hash for Python is:', hash('Python'))

When you run the program, the output will be:

Hash for 181 is: 181
Hash for 181.23 is: 530343892119126197
Hash for Python is: 2230730083538390373

Example 2: hash() for immutable tuple object?

The hash() method only works for immutable objects as tuple.

# tuple of vowels
vowels = ('a', 'e', 'i', 'o', 'u')

print('The hash is:', hash(vowels))

When you run the program, the output will be:

The hash is: -695778075465126279

How hash() works for custom objects?

As stated above, hash() method internally calls __hash__() method. So, any objects can override the __hash__() for custom hash values.

But for correct hash implementation, __hash__() should always return an integer. And, both __eq__() and __hash__() methods have to be implemented.

Below are the cases for correct __hash__() override.

Cases for custom hash implementation for objects
__eq__() __hash__() Description
Defined (by default) Defined (by default) If left as is, all objects compare unequal (except themselves)
(If mutable) Defined Should not be defined Implementation of hashable collection requires key's hash value be immutable
Not defined Should not be defined If __eq__() isn't defined, __hash__() should not be defined.
Defined Not defined

Class instances will not be usable as hashable collection.

__hash__() implicity set to None

Raises TypeError exception if tried to retrieve the hash.
Defined Retain from Parent __hash__ = <ParentClass>.__hash__
Defined Doesn't want to hash

__hash__ = None

Raises TypeError exception if tried to retrieve the hash.

Example 3: hash() for Custom Objects by overriding __hash__()

class Person:
    def __init__(self, age, name):
        self.age = age
        self.name = name

    def __eq__(self, other):
        return self.age == other.age and self.name == other.name

    def __hash__(self):
        print('The hash is:')
        return hash((self.age, self.name))

person = Person(23, 'Adam')
print(hash(person))

When you run the program, the output will be:

The hash is:
3785419240612877014

Note: You don't have to implement the __eq__() method for the hash as it is created by default for all objects.