GitHub: not-a-feature/blosum

PyPi: blosum

A small module for easy access to BLOSUM matrices without dependencies.

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The BLOcks SUbstitution Matrices (BLOSUM) are used to score alignments between protein sequences and are therefore mainly used in bioinformatics. Reading such matrices is not particularly difficult, yet most off the shelf packages are overloaded with strange dependencies. And why do we need to implement the same reader again if there is a simple module for that.

blosum offers a robust and easy-to-expand implementation without relying on third-party libraries.


Using pip / pip3:

pip install blosum

Or by source:

git clone
cd blosum
pip install .

How to use

Default BLOSUM

This package comes with the most commonly used BLOSUM matrices built in. You can choose from BLOSUM 45, 50, 62, 80 and 90.

To load a matrix:

import blosum as bl
matrix = bl.BLOSUM(62)

Custom Matrix

In addition, own matrices can be loaded. For this, the path is given as an argument.

import blosum as bl
matrix = bl.BLOSUM("path/to/blosum.file")

The matrices are required to have following format:

# One Header Line
   A  R  N  D
A  5 -2 -1 -2
R -2  7  0 -1
N -1  0  6  2
D -2 -1  2  7

Getting Values.

To get a value use:

val = matrix["AY"]

Or get the dictionary representation of the BLOSUM matrix.

blosum_dict = dict(matrix)

If the key cannot be found, the default value is returned. It is float("-inf"). It is possible to set a custom default score:

blosum_dict = dict(matrix)


Copyright (C) 2021 by Jules Kreuer - @not_a_feature
This piece of software is published unter the GNU General Public License v3.0

| Permissions      | Conditions                   | Limitations |
| ---------------- | ---------------------------- | ----------- |
| ✓ Commercial use | Disclose source              | ✕ Liability |
| ✓ Distribution   | License and copyright notice | ✕ Warranty  |
| ✓ Modification   | Same license                 |             |
| ✓ Patent use     | State changes                |             |
| ✓ Private use    |                              |             |

Go to to see the full version.


In addition to packages included in Python 3, this piece of software uses 3rd-party software packages for development purposes that are not required in the published version. Go to to see all dependencies and licenses.