Python - hashing JSON data structures.

README.md

Python - hashing JSON data structures

A function hash_json(data), accepting a structure loaded from json.load() and computing a hash.

Example

$ ./hashjson.py
Hash a.json: 8212462b8e9ce805cac2f0758127c5cfd7710baf
Hash b.json: 8212462b8e9ce805cac2f0758127c5cfd7710baf

JSON files a.json and b.json are loaded via load_json() and structures passed to hash_json().

a.json

{
	"apples": "oranges",
	"grapes": [
		1,
		2,
		3,
		4
	],
	"pears": {
		"one": 1,
		"two": 2,
		"three": 3
	}
}

b.json

{
	"pears": {
		"three": 3,
		"two": 2,
		"one": 1
	},
	"apples": "oranges",
	"grapes": [
		1,
		2,
		3,
		4
	]
}

hashjson.py

#!/usr/bin/env python3

import hashlib
import json
from typing import Any


def load_json(file_path):
    # open JSON file and parse contents
    fh = open(file_path, "r")
    data = json.load(fh)
    fh.close()

    return data


def hash_json(data: Any) -> str:
    def hasher(value: Any):
        if type(value) is list:
            # hash each item within the list
            for item in value:
                hasher(item)

            return

        if type(value) is dict:
            # work over each property in the dictionary, using a sorted order
            for item_key in sorted(value.keys()):
                # hash both the property key and the value
                hash.update(item_key.encode())
                hasher(value[item_key])

            return

        if type(value) is not str:
            value = str(value)

        hash.update(value.encode())

    # create new hash, walk given data and return result
    hash = hashlib.sha1()
    hasher(data)
    return hash.hexdigest()


def main():
    # import testing JSON files to Python structures
    a_json = load_json("a.json")
    b_json = load_json("b.json")

    # hash each JSON structure
    print(f"Hash a.json: {hash_json(a_json)}")
    print(f"Hash b.json: {hash_json(b_json)}")


if __name__ == "__main__":
    main()

I was looking for these exactly.
But I noticed a problem.

If you have for example a dict with 1 entry with key and value being the same string
and a list with 2 items that has that same string, both will return the same hash.

json_object = {
  'example': 'example',
}

json_list = [
  'example',
  'example',
]

hash_object = hash_json(json_object)
hash_list = hash_json(json_list)

print(f'Are hashes equal: {hash_object == hash_list}') # True

This could be solved by appending an arbitrary character before hashing a list to create a difference.