{"id":6209,"date":"2020-05-31T06:59:28","date_gmt":"2020-05-31T06:59:28","guid":{"rendered":"https:\/\/www.askpython.com\/?p=6209"},"modified":"2022-08-06T13:14:52","modified_gmt":"2022-08-06T13:14:52","slug":"python-hash-function","status":"publish","type":"post","link":"https:\/\/www.askpython.com\/python\/built-in-methods\/python-hash-function","title":{"rendered":"Using the Python hash() function"},"content":{"rendered":"\n

Hello everyone! In today’s article, we’ll be looking at Python’s in-built hash()<\/code> function. The Python hash()<\/code> function computes the hash value of a Python object. But the language uses this to a large extent.<\/p>\n\n\n\n

Let’s understand more about this function, using some examples!<\/p>\n\n\n\n

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Basic Syntax of Python hash()<\/h2>\n\n\n\n

This function takes in an immutable<\/em> Python object<\/a>, and returns the hash value of this object.<\/p>\n\n\n

\nvalue = hash(object)\n<\/pre><\/div>\n\n\n
Remember that the hash value is dependent on a hash function, (from __hash__()<\/code>), which hash()<\/code> internally calls. This hash function needs to be good enough such that it gives an almost random distribution.<\/p>\n\n\n\n
Well, why do we want a hash function to randomize its values to such a large extent? This is because we want the hash function to map almost every key to a unique value.<\/p>\n\n\n\n
If your values are randomly distributed, there will be very little chance of two different keys being mapped to the same value, which is what we want!<\/p>\n\n\n\n
Now, let’s look at the hash()<\/code> function in use, for simple objects like integers, floats and strings.<\/p>\n\n\n\n
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Using the hash() function – Some Examples<\/h2>\n\n\n
\nint_hash = hash(1020)\n\nfloat_hash = hash(100.523)\n\nstring_hash = hash("Hello from AskPython")\n\nprint(f"For {1020}, Hash : {int_hash}")\nprint(f"For {100.523}, Hash: {float_hash}")\nprint(f"For {'Hello from AskPython'}, Hash: {string_hash}")\n<\/pre><\/div>\n\n\n
Output<\/strong><\/p>\n\n\n
\nFor 1020, Hash : 1020\nFor 100.523, Hash: 1205955893818753124\nFor Hello from AskPython, Hash: 5997973717644023107\n<\/pre><\/div>\n\n\n
As you can observe, integers have the same hash value as their original value. But the values are obviously different for the float and the string objects.<\/p>\n\n\n\n
Now, it won’t be very safe if the same object (except integers\/floats) always has the same hash value. So, if you run the above snippet again, you’ll notice different values!<\/p>\n\n\n\n
For example, this is my output when I run the same snippet for the second time.<\/p>\n\n\n
\nFor 1020, Hash : 1020\nFor 100.523, Hash: 1205955893818753124\nFor Hello from AskPython, Hash: -7934882731642689997\n<\/pre><\/div>\n\n\n
As you can see, the value is changed for the string! This is a good thing because it prevents the same object from being potentially accessed by someone! The hash value remains constant only until the duration of your program.<\/p>\n\n\n\n
After that, it keeps changing every time you run your program again.<\/p>\n\n\n\n
Why cannot we use hash() on mutable objects?<\/h2>\n\n\n\n
Now, remember that we mentioned earlier about hash()<\/code> being used only on immutable<\/em> objects. What does this mean?<\/p>\n\n\n\n
This means that we cannot use hash()<\/code> on mutable objects like lists, sets, dictionaries, etc.<\/p>\n\n\n
\nprint(hash([1, 2, 3]))\n<\/pre><\/div>\n\n\n
Output<\/strong><\/p>\n\n\n
\nTypeError: unhashable type: 'list'\n<\/pre><\/div>\n\n\n
Why is this happening? Well, it would be troublesome for the program to keep changing the hash value every time the value of a mutable object changes.<\/p>\n\n\n\n
This will make it very time consuming to keep updating the hash value again. If you do this, then Python needs to take a lot of time to keep referring to the same object, since the references will keep changing!<\/p>\n\n\n\n
Due to this, we cannot hash mutable objects using hash()<\/code>, since they only have a single value, which is hidden from us, so that the program can internally keep a reference to it.<\/p>\n\n\n\n
However, we can<\/em> use hash()<\/code> on an immutable tuple. This is a tuple<\/a> that consists of only immutable objects, like ints, floats, etc.<\/p>\n\n\n
\n>>> print(hash((1, 2, 3)))\n2528502973977326415\n\n>>> print(hash((1, 2, 3, "Hello")))\n-4023403385585390982\n\n>>> print(hash((1, 2, [1, 2])))\nTraceback (most recent call last):\n  File "<stdin>", line 1, in <module>\nTypeError: unhashable type: 'list'\n<\/pre><\/div>\n\n\n
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Using hash() on a Custom Object<\/h2>\n\n\n\n
Since the default Python hash()<\/code> implementation works by overriding the __hash__()<\/code> method, we can create our own hash()<\/code> method for our custom objects, by overriding __hash__()<\/code>, provided that the relevant attributes are immutable.<\/p>\n\n\n\n
Let’s create a class Student<\/code> now.<\/p>\n\n\n\n
We’ll be overriding the __hash__()<\/code> method to call hash()<\/code> on the relevant attributes. We will also be implementing the __eq__()<\/code> method, for checking equality between the two custom objects.<\/p>\n\n\n
\nclass Student:\n    def __init__(self, name, id):\n        self.name = name\n        self.id = id\n\n    def __eq__(self, other):\n        # Equality Comparison between two objects\n        return self.name == other.name and self.id == other.id\n\n    def __hash__(self):\n        # hash(custom_object)\n        return hash((self.name, self.id))\n\nstudent = Student('Amit', 12)\nprint("The hash is: %d" % hash(student))\n\n# We'll check if two objects with the same attribute values have the same hash\nstudent_copy = Student('Amit', 12)\nprint("The hash is: %d" % hash(student_copy))\n<\/pre><\/div>\n\n\n
Output<\/strong><\/p>\n\n\n
\nThe hash is: 154630157590\nThe hash is: 154630157597\n<\/pre><\/div>\n\n\n
We can indeed observe the hash of our custom object. Not only that; two different objects even with the same attribute values, have different hash values!<\/p>\n\n\n\n
This is indeed what we want to expect from a hash function, and hash()<\/code> has successfully given us that!<\/p>\n\n\n\n
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Conclusion<\/h2>\n\n\n\n
We learned about using the Python hash()<\/code> function. This is very useful for the program to maintain references to each object, using a special integer value.<\/p>\n\n\n\n
We also saw how we could make hash()<\/code> work on custom objects, provided it’s attributes are immutable.<\/p>\n\n\n\n
References<\/h2>\n\n\n\n
  • JournalDev article on Python hash() function<\/li><\/ul>\n\n\n\n
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