What are Hashable and Unhashable objects?

An object is hashable if its hash value never changes during its lifetime. All of Python’s immutable built-in objects(such as strings and tuples) are hashable, while mutable containers (such as lists or dictionaries) are unhashable.

Objects which are instances of user-defined classes are hashable by default. The immutability of an object enables Python to derive its hash value.

Hashability makes an object usable as a dictionary key and a set member because these data structures use the hash value internally.

How to check if an object is hashable or not?

Python built-in data-structure set to the rescue. If you try to add an unhashable object into a set, it will throw a TypeError exception

def check_hashable(an_object):
    try:
        set().add(an_object)
        print("The object is hashable")
    except TypeError:
        print("The object is unhashable")

>>> check_hashable(100)
The object is hashable
>>> check_hashable("mystring")
The object is hashable
>>> check_hashable([1, 2, 3, 4])
The object is unhashable
>>> check_hashable((1, 2, 3, 4))
The object is hashable
>>> check_hashable({"blog-name": "realnitinworks", "author": "Nitin"})
The object is unhashable

We try to add the object to a set inside the try...except block. If it throws a TypeError exception, the object is not hashable, otherwise, it is hashable.

Checking hashability using Hashable class

The Hashable class is available in collections.abc - An abstract base class for containers

from collections.abc import Hashable

def check_hashable(an_object):
    if isinstance(an_object, Hashable):
        print("The object is hashable")
    else:
        print("The object is unhashable")

>>> check_hashable(100)
The object is hashable
>>> check_hashable("mystring")
The object is hashable
>>> check_hashable([1, 2, 3, 4])
The object is unhashable

In the above code, we use the built-in isinstance function to check if the passed-in object is an instance of Hashable or not.

Conclusion

We have seen what does it means to be hashable or unhashable with respect to an object. I also introduced two methods to check if an object is hashable or not.