{"id":2593,"date":"2021-10-19T15:38:13","date_gmt":"2021-10-19T15:38:13","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2593"},"modified":"2025-03-31T09:00:50","modified_gmt":"2025-03-31T09:00:50","slug":"python-__hash__","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-__hash__\/","title":{"rendered":"Python __hash__"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python hash() function and how to override the __hash__<\/code> method in a custom class.<\/p>\n\n\n\n

Introduction to the Python hash function #<\/a><\/h2>\n\n\n\n

Let’s start with a simple example.<\/p>\n\n\n\n

First, define the Person<\/code> class<\/a> with the name<\/code> and age<\/code> attributes:<\/p>\n\n\n

class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create two instances of the Person<\/code> class:<\/p>\n\n\n
p1 = Person('John'<\/span>, 22<\/span>)\np2 = Person('Jane'<\/span>, 22<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, show the hashes of the p1<\/code> and p2<\/code> objects:<\/p>\n\n\n
print(hash(p1))\nprint(hash(p2))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
110373112736<\/span>\n110373572343<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Here’s the complete script:<\/p>\n\n\n
class Person:\n    def __init__(self, name, age):\n        self.name = name\n        self.age = age\n        \np1 = Person('John', 22)\np2 = Person('Jane', 22)        \n\nprint(hash(p1))\nprint(hash(p2))<\/code><\/span><\/pre>\n\n\n
Try it<\/a><\/p>\n\n\n\n
The hash()<\/code> function accepts an object and returns the hash value as an integer. When you pass an object to the hash()<\/code> function, Python will execute the __hash__<\/code> special method of the object.<\/p>\n\n\n\n
It means that when you pass the p1<\/code> object to the hash()<\/code> function:<\/p>\n\n\n
hash(p1)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python will call the __hash__<\/code> method of the p1<\/code> object:<\/p>\n\n\n
p1.__hash__()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If a class overrides the __eq__<\/code> method, the objects of the class become unhashable. This means that you won’t able to use the objects in a mapping type. For example, you will not able to use them as keys in a dictionary<\/a> or elements in a set<\/a>.<\/p>\n\n\n\n
The following Person<\/code> class implements the __eq__<\/code> method:<\/p>\n\n\n
class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n    def<\/span> __eq__<\/span>(self, other)<\/span>:<\/span>\n        return<\/span> isinstance(other, Person) and<\/span> self.age == other.age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you attempt to use the Person<\/code> object in a set, you’ll get an error. For example:<\/p>\n\n\n
members = {\n    Person('John'<\/span>, 22<\/span>),\n    Person('Jane'<\/span>, 22<\/span>)\n}<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python issues the following error:<\/p>\n\n\n
TypeError: unhashable type: 'Person'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Also, the Person’s object loses hashing because if you implement __eq__<\/code>, the __hash__<\/code> is set to None<\/code>. For example:<\/p>\n\n\n
hash(Person('John'<\/span>, 22<\/span>))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
TypeError: unhashable type: 'Person'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To make the Person<\/code> class hashable, you also need to implement the __hash__<\/code> method:<\/p>\n\n\n
class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n    def<\/span> __eq__<\/span>(self, other)<\/span>:<\/span>\n        return<\/span> isinstance(other, Person) and<\/span> self.age == other.age\n\n    def<\/span> __hash__<\/span>(self)<\/span>:<\/span>\n        return<\/span> hash(self.age)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, you have the Person<\/code> class that supports equality based on age<\/code> and is hashable.<\/p>\n\n\n\n
class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self._age = age\n\n    @property<\/span>\n    def<\/span> age<\/span>(self)<\/span>:<\/span>\n        return<\/span> self._age\n\n    def<\/span> __eq__<\/span>(self, other)<\/span>:<\/span>\n        return<\/span> isinstance(other, Person) and<\/span> self.age == other.age\n\n    def<\/span> __hash__<\/span>(self)<\/span>:<\/span>\n        return<\/span> hash(self.age)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
To make the Person<\/code> work well in data structures like dictionaries, the hash of the class should remain immutable. To do it, you can make the age<\/code> attribute of the Person<\/code> class a read-only property<\/a>:<\/p>\n\n\n
By default, the __hash__<\/code> uses the object’s identity and the __eq__<\/a><\/code> returns True<\/code> if two objects are the same. To override this default behavior, you can implement the __eq__<\/code> and __hash__<\/code>.<\/p>\n\n\n\n