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Semantic versioning and semv

It is somewhat clear that software releases should carry a version like v3 or v1.1 or something. But this often seems more or less arbitrary, and you might feel that just counting git-commits or adding the current date would do equally well. After all, these would be easy to create automatically and once set up, you would never have to think about versioning again.

In fact, I used to think that way. I don't think so anymore though. More yet, I believe that versions are a key ingredient in decoupling different software components. At least if the versions are done right.

Why do we have versions? Dependencies and releases

Let's take a step back and think for a moment: Why do people create version identifiers for their software in the first place? Apart from a number of marketing reasons ("Wow, look at this, we just did a major release. Expect lots of cool new features!"), there are actually very valid technical reasons too.

Versions are a means of communicating changes to your users. Of course, this only really applies to technical users who want to use your software as an API or a library (and this post will mostly focus on libraries with a few mentions of APIs). Ultimately, what you are telling your users is: "This is a new version, expect things to change". Based on this information, your users can decide if and when they want to switch to using the new software. Maybe, they say to themselves: "Well, everything is working fine for us. We don't want to respond to things changing. We'll just stick with the old version." And that's of course a perfectly valid decision.

In this example, I assumed that version identifiers can only carry this one message: "Things may change." Is there more, that could be conveyed? It depends.

How should we assign version identifiers?

There are in fact a number of ways how "things may change". Hence, it makes sense to broaden the vocabulary for communicating with your users. Different versioning schemes attempt to do that. For example, one can use dates as version numbers to communicate when the release was published. If a user than finds that they are running their software on release 1998-06-01.1, they know that it might be a good idea to think about upgrading eventually.

Some software uses version numbers like 1.3 or 2.0. Most people somewhat intuitively understand that version 1.3 was before version 2.0 and that incrementing the part before the dot indicates something more significant than incrementing the part after the dot. However, what do they indicate? For many software projects the distinction is somewhat arbitrary.

So what could we as developers attempt to tell our users with a version identifier? Can we try to be more specific about the kind of change that happend between the two versions? Semantic versioning attempts to do just that. It classifies possible changes between software versions into three classes

  1. Major changes are anything that breaks the interface of the software. In other words, after a major change, a downstream user (an imaginary downstream user who uses all features) must change the way they use the software. Of course in reality not everybody has to change the way they use the software, but if a major change is communicated it is a good idea for users to check if the changes will affect them.
  2. Minor changes add functionality. Typically this will be a new feature. Minor changes offer a chance to use the software differently—after all you could now use the new feature—but whatever you did before will still work fine.
  3. Patch changes mostly affect the inner workings of the software. These are usually bug fixes or performance improvements. There is no reason to change your code really, but you can expect things to just work a little more smoothly and efficiently.

With semantic versioning, we encapsulate these three kinds of changes into a version number: vX.Y.Z, where X=Major version, Y=Minor version and Z=Patch version. Often, there are additional qualifiers, for example in python, there are several postfixes to indicate things like release candidates in which the actual version is altered but the major, minor and patch components remain the same.

Semantic versions are useful, because they mean that you know in advance what's going to happen if you upgrade to a new version. You can also specify that you want a version between v1.3.0 but less then v2.0.0. For example in a python requirements file, you would state this as ~=1.3. If you rely on a fix that was introduced in version 1.3.2, you could write >=1.3.2,<2.0. This allows freely installing packages without risking to get something that will break your code.

A brief remark on APIs: For APIs, hosting multiple concurrent versions can be a lot of hassle. It is therefore common, to only host separate versions for each major version, but apply minor and patch releases as they come. This tends to be an ok compromise between compatibility and hosting many separate API versions.

Sounds like this could be done automatically...

You may be wondering if semantic versioning could be done automatically. And the answer is both, yes and no. "No", because no version identification tool can know if the changes in the code are implementing a new feature or fixing a bug. "Yes", because it would in principle be possible to convey that information through commit messages. Once commits contain the information about what kind of change has been committed, a versioning tool could parse the information from the commit messages and generate new software versions based on this information.

Do you want to do this automatically? Although I strongly recommend using semantic versions, I don't think that all packages must be automatically versioned. Specifically, if a dedicated person is responsible for managing the features that should go into the next release and planning when that release would be, then it might be a valid decision to leave the version number to that person. After all, that person might feel that it is a good idea to group multiple breaking changes together into one big major release and add features incrementally in separate releases (I would also agree with that person). However, planning releases like this is quite a lot of work and it might not be worth it for a small open source project. Furthermore, even a dedicated release manager might still decide to generate the version identifiers automatically.

If your project or organization is to small to have someone who specifically manages releases, then automatic semantic versioning is likely great. For example, one could setup a continuous integration system to create a new release every time a pull request gets merged—of course this setup would need to automatically generate version numbers.

Introducing semv, a semantic version commit parser

In the previous paragraph, I linked two software tools that perform semantic releases above. One is the "original" semantic release npm package. However, my main language is python and the npm package only releases to npm. I therefore also linked the package python-semantic-release which performs something similar for the python world of pip and

I personally found the semantic release packages to attempt to do too much. They create new versions, but they also add commits with version specifiers and potentially even push them to GitHub without you knowing. In addition, they upload packages to a central repository and make releases on GitHub. Although this can be great, it can also be in your way—a lot! Furthermore, when playing with python-semantic-release, I felt that I would often mess up the commit format or (worse yet!) forget to mark a breaking change correctly. Often I found out about my mistake after python-semantic-release had done its thing. As a result, I had to even roll back an accidentally released minor version.

What I want is a tool that simply determines the next release number but leaves the releasing to my discretion. In fact, I want a tool that does not change anything by itself and requires minimal configuration. After I didn't find that tool, I wrote it myself and called it semv like semantic version parser. I can run semv as part of an automatic release process or as a commit-msg hook. It has absolutely no required configuration (although it can be configured via pyproject.toml) and it will never change anything about your repository.

Using semv is dead simple:

  $ semv

This command will print the version that your software should get if you would want to release from the current commit. There is just one requirement: If you decide to actually do a release, you should tag it with a message of the form vX.Y.Z. These tags will be used by semv to analyze only commits since the last version tag.

One issue with automatic semantic releases is, that people may accidentally forget to mark a change as "breaking". As a result, the new version would be marked a minor (or patch) release and people would install that breaking change without much consideration. Of course afterwards their dependent software is broken. Stephan Bönnemann, one of the authors of the original semantic release npm package, suggests an additional step to verify a version identifier: Running the tests of the previous version on a new release candidate. If the previous version's tests fail, then the release is almost certainly broken.

In semv I added an option to include additional verification checks. However, only one such check is implemented so far: running the previous version's tests against the new release candidate (using tox).


In this post, I pointed out that version identifiers are a way for api and library developers to communicate changes to their users. I further argued that semantic versioning is a versioning schema that compactly conveys the most important information about the changes that go along with a software release. I then introduced semv as a tool to automatically create version identifiers by parsing commit messages.

Do you do semantic versioning in your own projects? Do you find it useful? Do you use any tooling for it? If so what are the tools you use? Do you think that the "no write" policy of semv is a good thing? Please let me know in the comments.