Python Hashlib Module

The hashlib module provides secure hash functions such as SHA-256 and MD5.

import hashlib

Hash functions turn data into a fixed-length digest. A small change in the input creates a very different digest.

Creating a hash

You usually start by choosing an algorithm and passing bytes.

import hashlib

digest = hashlib.sha256(b'hello world')
print(digest.hexdigest())

b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

Strings must be encoded to bytes first:

import hashlib

message = 'hello world'
digest = hashlib.sha256(message.encode('utf-8')).hexdigest()
print(digest)

b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

Updating incrementally

Hashes can be updated in chunks.

hasher = hashlib.sha256()
hasher.update(b'hello ')
hasher.update(b'world')
print(hasher.hexdigest())

b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

Comparing algorithms

Different algorithms produce different digest sizes.

print(hashlib.md5(b'abc').hexdigest())
print(hashlib.sha1(b'abc').hexdigest())
print(hashlib.sha256(b'abc').hexdigest())

900150983cd24fb0d6963f7d28e17f72
a9993e364706816aba3e25717850c26c9cd0d89d
ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad

Hashing a file

Read files in chunks so large files do not need to be loaded into memory all at once.

import hashlib
from pathlib import Path

path = Path('example.txt')
path.write_text('hello world')

hasher = hashlib.sha256()
with path.open('rb') as file:
    for chunk in iter(lambda: file.read(8192), b''):
        hasher.update(chunk)

print(hasher.hexdigest())
path.unlink()

b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

Relevant links

• Module: pathlib
• Cheatsheet: Reading and Writing Files
• open()