How to write a hash function in python

Concerning potential hash collisions: Yeah, that's a C thing. Please refer to the glossary: It's also used to access dict and set elements which are implemented as resizable hash tables in CPython.

How to write a hash function in python

August 29, This post describes how dictionaries are implemented in the Python language. Dictionaries are indexed by keys and they can be seen as associative arrays. Hash tables Python dictionaries are implemented using hash tables.

It is an array whose indexes are obtained using a hash function on the keys. The goal of a hash function is to distribute the keys evenly in the array. A good hash function minimizes the number of collisions e.

how to write a hash function in python

Python does not have this kind of hash function. Its most important hash functions for strings and ints are very regular in common cases: The hash function for strings in Python is defined as: Here we assume we are using a bit machine.

This makes the computation of the slot index fast. The probability to find an empty slot is high due to the resizing mechanism described below. This means that having a simple computation makes sense in most of the cases.

We can see that the Python hash function does a good job when the keys are consecutive which is good because it is quite common to have this type of data to work with. We could use a linked list to store the pairs having the same hash but it would increase the lookup time e.

The next section describes the collision resolution method used in the case of Python dictionaries. Open addressing Open addressing is a method of collision resolution where probing is used. A quadratic probing sequence is used to find a free slot.

The code is the following: A detailed explanation of the probing mechanism can be found at the top of the file. Dictionary C structures The following C structure is used to store a dictionary entry: The hash, key and value are stored.

PyObject is the base class of the Python objects. A slot is marked dummy when a key pair is removed. I removed some of the lines and converted the C code to pseudocode to concentrate on the key concepts.

It checks if the key is a string and calculates the hash or reuses the one cached if it exists.The @classmethod form is a function decorator – see the description of function definitions in Function definitions for details..

It can be called either on the class (such as C.f()) or on an instance (such as C().f()).The instance is ignored except for its class.

If a class method is called for a derived class, the derived class object is passed as the implied first argument. Return the hash value of the object (if it has one). Hash values are integers. They are used to quickly compare dictionary keys during a dictionary lookup.

Numeric values that compare equal have the same hash value (even if they are of different types, as is the case for 1 and ). The hash table should be engineered such that each slot in the table is equally likely to be picked by the hash function (to avoid collision) and to have a sub linear time complexity.

[math] P(X = index) = \dfrac{1}{k} [/math] where k is the table size. We are currently dealing with hash function in my class.

Our instructor asked us to a hash function on the internet to compare to the two we have used in our code. Getting the same hash of two separating files means that there is a high probability the contents of the files are identical, even though they have different names.

MD5 File Hash in Python The code is made to work with Python and higher (including Python 3.x). 2. Built-in Functions¶. The Python interpreter has a number of functions and types built into it that are always available.

how to write a hash function in python

They are listed here in alphabetical order.

Python hash() function on strings - Stack Overflow