qsum: Python checksumming toolkit

Intuitive and extendable checksumming for python objects

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Goals

• Provide a checksumming toolkit for python with out of the box support for common types
• Architect a framework for implementing customized checksumming logic
• Produce high quality checksums with extraordinarily low collision rates
• Build a toolkit for using and manipulating checksums
• Test it all with 100% coverage and support python 3.7, 3.8, 3.9, 3.10, 3.11 and 3.12

Where to get it

Source code is available on github: https://github.com/QCoding

Install with conda:

# from conda forge: https://anaconda.org/conda-forge/qsum
conda install qsum

Install with pip:

# from PyPI: https://pypi.org/project/qsum/
pip install qsum

How to use it

# Functional Interface
from qsum import checksum
checksum('abc')

# Class Interface
from qsum import Checksum
Checksum('abc').checksum_bytes

Design

• QSUM CHECKSUM = TYPE PREFIX + DATA CHECKSUM
  • The first two bytes of every checksum represent the type and will be referred to as the 'type prefix'
  • The rest of the checksum in a digest of the byte representation of the object and will be refered to as the 'data checksum'

Relationship to __hash__

• Respect the same contract as __hash__ with regards to: 'The only required property is that objects which compare equal have the same hash value'
• Do not salt hash values (unless requested) and maintain stability in checksums throughout python sessions and versions along with releases of this package
• PYTHONHASHSEED should have no effect on checksums
• Provide significantly longer checksums than __hash__ which 'is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds'
• Represent all checksums as bytes but provide a toolkit to view more human readable formats like hexdigests
• Base checksums on object contents and permit the calculation of checksums on mutable objects

Adding Salt

• By default the environment is not included in the checksum but individual package versions can be included if the package name is added via the depends_on argument
• To include the entire python environment in the checksum:

  from qsum import checksum, DependsOn
  checksum('abc', depends_on=DependsOn.PythonEnv)
  

Type Support

• The great majority of Built-in Types including collections are checksummable
  • bool, int, float, complex, str, bytes, tuple, list, dict, set, deque, etc.
• Common types have registered type prefixes which can be used to recover the type from the checksum

Custom Containers

• Custom container classes that inherit from common python containers (E.g. tuple, list, set, dict) are checksummable
• The class name is not recoverable from the type prefix but will be added as salt to the data checksum to prevent collisions

Functions and Modules

• Functions are checksummed based on a combination of their source code, attributes and module location
• Modules are checksummed simply based on the hash of their source code

Files

• When passed an open file handle qsum will include all the bytes of the file in the checksum calculation

References

Wikipedia Checksum

Python Hashlib

Python __hash__

What Happens When You Mess With Hashing In Python