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Versioning and formatting your Python code

8 minute read #python, #github, #tutorial, #black, #versioneer, #semver

This is part 2 in the Creating an open source Python project from scratch series. If you haven’t read the previous parts you may want to go back and check those out.

In this post, we will cover a few project hygiene things that we may want to put into place to make our lives easier in the future.


At this point we should put some thought into versioning our library. When we make changes to our code we will want to release a new version, but what kind of naming scheme should we use for our version labels?

There are several commonly used versioning standards used in open-source software. The two that you will see most often are Semantic Versioning or SemVer for short, and Calendar Versioning or CalVer for short. The first increments the version numbers based on what changes are made to the software, the second increments the version number based on when those changes were made.

SemVer is commonly used when a project has an API which is being presented to the user, the developer can use the version information to communicate to the user whether or not new versions will impact their use of the library. CalVer is more commonly used for projects which do not necessarily provide an API to the user, an example may be a set of configuration files for a system which changes over time.


In this example, we are going to use SemVer. SemVer aims to communicate to your users what has changed between versions by incrementing one of three numbers separated by a dot.

Let’s discuss each number but starting right to left, as that will take us from the smallest change to the biggest change. The righthand number is referred to as the bugfix version and should be incremented if we fix a bug in our library. For example, if there is a small problem that is breaking things for users in version 1.0.0 and we fix it the next release should be 1.0.1.

The middle number is referred to as the minor version number. This should be incremented if we add a new feature to our library. Existing users should be able to continue using the library exactly as they always had done regardless of the new feature. Also when incrementing this number we should reset the bugfix number to 0. For example, if we added a new function to our library, but left is_number totally alone, this is a minor modification and the next release should be 1.1.0.

The left number is the major version number. This should be incremented if we have made a change which will break things for our users. After this version, they cannot just continue as usual and will probably have to read the documentation and change their behavior accordingly. We also reset both other numbers to 0. For example, if we modified is_number to return the strings 'yes' and 'no' instead of the booleans True and False this would be a breaking change and we should release 2.0.0.

In our setup.py we have set the initial version to 0.0.1. When the major version of a library is 0 this means it is in development mode and should be considered unstable. Generally in this mode, the bugfix version remains the same but the minor version is incremented for both major and minor changes, meaning anything could break at any time. Ideally once a library is “finished” and the functionality is stable you should move on to version 1.0.0 and adopt SemVer more strictly, however you will notice in the wild that many well-used projects have not yet had a version 1.0.0 release. This is because many open source projects are run by volunteers. Folks with limited time to work on projects do not want to commit to a stable API.

For more information on this schema see the Semantic Versioning website.

Automatic version detection

As we are using git for our version control we also have the ability to tag specific commits with our version numbers. However it is easy to forget to update our setup.py file when creating a tag, so it would be great to enable our setup to automatically detect the current version from the git information.

To do this we can use versioneer which is a Python module that you package with your project. Instead of explicitly setting your version in setup.py versioneer gets it dynamically from the version control.

pip install versioneer

For versioneer to correctly identify our package version we need to provide some configuration. This is done by adding a [versioneer] section to a file called setup.cfg. This file is used for configuring many Python tools so let’s create it.

VCS = git
style = pep440
versionfile_source = is_number/_version.py
versionfile_build = is_number/_version.py
tag_prefix =
parentdir_prefix =

In our config, we have specified that we are using git for our version control and that our version numbers should follow PEP 440.

We also specify where our version file should live inside our package and that we are not using any prefixes.

Next, we need to install the versioneer module into our project.

versioneer install

Running this command will create a few new files for us in our project and also ask us to make some changes to our setup.py file. We need to import versioneer in the setup, replace the version keyword argument with versioneer.get_version() and add a new argument cmdclass=versioneer.get_cmdclass().

import setuptools
import versioneer

with open("README.rst", "r") as fh:
    long_description = fh.read()
with open("requirements.txt", "r") as fh:
    requirements = [line.strip() for line in fh]

    author="Jacob Tomlinson",
    description="A Python library to determine if something is a number.",
        "Programming Language :: Python :: 3",
        "License :: OSI Approved :: MIT License",
        "Operating System :: OS Independent",

We should also go and have a look at the __init__.py file where our code lives as versioneer has added some extra lines in there. These lines use versioneer to set a __version__ variable in the root scope of the file, this is common practice for Python libraries.

from .is_number import is_number

from ._version import get_versions
__version__ = get_versions()['version']
del get_versions

It has also created a versioneer.py, MANIFEST.in and .gitattributes file in the root of the project and a _version.py file in our library folder.

The versioneer.py and _version.py files contain the versioneer module code for calculating the version number from the git information.

The .gitattributes file tells git to run these files when an archive is being created during publishing, this will replace the contents of _version.py with a static version. This is important as the git history will not be included in the archive.

The MANIFEST.in file tells setuptools which files to include when publishing our package. We will come on to publishing in another post but as versioneer has created this file for us let’s also put some other things into this file.

include README.rst
include LICENSE
include requirements.txt
graft is_number
recursive-exclude * *.py[co]
include versioneer.py
include is_number/_version.py

Here we are ensuring that other files like our README.rst and requirements.txt are going to be included in our published package, this is important because our setup reads them in. We set graft is_number to ensure all files in our library are included but then exclude all compiled .pyc and .pyo files as we do not want those to be packaged.

Now let’s test that we can get the version of our package with versioneer.

>>> import is_number
>>> is_number.__version__

We can see that we are getting a version number here, but it looks a little unusual. This is because we haven’t created any git tags yet and we have uncommitted changes in our repo.

Let’s commit all of our changes and tag our first version.

git add -A
git commit -m "Add versioneer"
git tag 0.0.1

Now if we run the same code again we should see the correct version.

>>> import is_number
>>> is_number.__version__

Formatting (Black)

A good practice for any open source project is to have a standard for formatting your code. Python has PEP 8 which is a style guide with recommendations on how to format your code. This guide is great and has lots of good advice and there are many linting tools, flake8 for example, to check that your code complies with this standard.

It has become commonplace for projects to use an automatic formatting tool such as Black. Tools like Black will take a codebase and format it to comply with PEP 8, but it will also make some opinionated decisions when it is unclear what should be done. This results in code being formatted consistently across the codebase and reduces the amount of time developers will spend arguing about “correct” formatting.

You may not be 100% happy with the opinionated decisions that Black makes, but I encourage you to just embrace them as the reduction in mental overhead is worth the compromise. I even suggest you set your code editor to run Black automatically when you save a file.

To run black on your codebase you need to install it.

pip install black

Then you can format individual files or your entire project.

# Format everything in the current directory
black .

This will even format the versioneer files that were created, which I don’t think is a terrible thing. Let’s commit this formatting and move on.

git add -A
git commit -m "Apply Black"


In this post we have covered:

In future posts we will cover:

More in this series

This was part 2 in the Creating an open source Python project from scratch series.

Next up, Publishing open source Python packages on GitHub, PyPI and Conda Forge

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