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
hash() Syntax
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)
hash() Return Value
The hash() method returns the hash value of an object.
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
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
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. |
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
The hash is: 3785419240612877014
Note: You don't have to implement __eq__() method for the hash as it is created by default for all objects.
