Python __hash__

Summary: in this tutorial, you’ll learn about the Python hash() function and how to override the __hash__ method in a custom class.

Introduction to the Python hash function

Let’s start with a simple example.

First, define the Person class with the name and age attributes:

class Person: def __init__(self, name, age): self.name = name self.age = age
Code language: Python (python)

Second, create two instances of the Person class:

p1 = Person('John', 22) p2 = Person('Jane', 22)
Code language: Python (python)

Third, show the hashes of the p1 and p2 objects:

print(hash(p1)) print(hash(p2))
Code language: Python (python)

Output:

110373112736 110373572343
Code language: Python (python)

The hash() function accepts an object and returns the hash value as an integer. When you pass an object to the hash() function, Python will execute the __hash__ special method of the object.

It means that when you pass the p1 object to the hash() function:

hash(p1)
Code language: Python (python)

Python will call the __hash__ method of the p1 object:

p1.__hash__()
Code language: Python (python)

By default, the __hash__ uses the object’s identity and the __eq__ returns True if two objects are the same. To override this default behavior, you can implement the __eq__ and __hash__.

If a class overrides the __eq__ 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 or elements in a set.

The following Person class implements the __eq__ method:

class Person: def __init__(self, name, age): self.name = name self.age = age def __eq__(self, other): return isinstance(other, Person) and self.age == other.age
Code language: Python (python)

If you attempt to use the Person object in a set, you’ll get an error. For example:

members = { Person('John', 22), Person('Jane', 22) }
Code language: Python (python)

Python issues the following error:

TypeError: unhashable type: 'Person'
Code language: Python (python)

Also, the Person’s object loses hashing because if you implement __eq__, the __hash__ is set to None. For example:

hash(Person('John', 22))
Code language: Python (python)

Error:

TypeError: unhashable type: 'Person'
Code language: Python (python)

To make the Person class hashable, you also need to implement the __hash__ method:

class Person: def __init__(self, name, age): self.name = name self.age = age def __eq__(self, other): return isinstance(other, Person) and self.age == other.age def __hash__(self): return hash(self.age)
Code language: Python (python)

Now, you have the Person class that supports equality based on age and is hashable.

To make the Person work well in data structures like dictionaries, the hash of the class should remain immutable. To do it, you can make the age attribute of the Person class a read-only property:

class Person: def __init__(self, name, age): self.name = name self._age = age @property def age(self): return self._age def __eq__(self, other): return isinstance(other, Person) and self.age == other.age def __hash__(self): return hash(self.age)
Code language: Python (python)

Summary

  • By default, __hash__ uses the id of objects and __eq__ uses the is operator for comparisons.
  • If you implement __eq__, Python sets __hash__ to None unless you implement __hash__.
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