Python hash() - VietMX's Blog

Table of Contents

1. hash() Syntax
2. hash() Parameters
3. hash() Return Value
4. Example 1: How hash() works in Python?
5. Example 2: hash() for immutable tuple object?
6. How does hash() work for custom objects?
- 6.1. Example 3: hash() for Custom Objects by overriding __hash__()

In this tutorial, we will learn about the Python hash() method with the help of examples.

The hash() method returns the hash value of an object if it has one. Hash values are just integers that are used to compare dictionary keys during a dictionary look quickly.

Example

text = 'Python Programming'

# compute the hash value of text
hash_value = hash(text)
print(hash_value)

# Output: -966697084172663693

1. hash() Syntax

The syntax of hash() method is:

hash(object)

2. hash() Parameters

The hash() method takes a single parameter:

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

3. hash() Return Value

The hash() method returns the hash value of an object.

4. 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'))

Output

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

5. Example 2: hash() for immutable tuple object?

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

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

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

Output

The hash is: -695778075465126279

6. How does hash() work for custom objects?

As stated above, hash() method internally calls __hash__() method. So, any objects can override __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.

__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.

6.1. 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))

Output