[Python-Dev] Python version numbers (original) (raw)

Chris Angelico rosuav at gmail.com
Tue Apr 3 10:15:01 EDT 2018

On Tue, Apr 3, 2018 at 10:51 PM, Paul G <paul at ganssle.io> wrote:

Breaking this off from the pickle thread because it seems unrelated:

On 04/02/2018 06:57 PM, Lukasz Langa wrote: I think we need to get past thinking about "Python 2" vs. "Python 3". This frame of mind creates space for another mythical release of Python that will break all the compatibilities, something we promised not to do. A moving backward compatibility window that includes the last release still under security fixes seems like a good new framework for this. Maybe this has already been discussed ad nauseum, but is the idea here that Python will stay on Python 3.x, but also start breaking backwards compatibility with old versions? That would seem to be a violation of semantic versioning.

Compatibility can be broken in small ways by minor versions. For instance, if you have a function "def await(x):", that will work perfectly on older Pythons, but will fail now that 'await' is a keyword. Does that require a major version bump? The pickle situation was one of changing a default, and if you need compatibility with ancient versions, you can specify protocol 0. By maintaining a sane default, we encourage people to use that rather than specifying protocol=-1 and breaking compatibility immediately when a new protocol is created.

The expectation is that a minor version bump generally won't break things, but you test your code anyway, in case it does. (A revision release should never break anything that wasn't already broken, eg depending on a bug.) A change to the default pickle protocol seems like a fairly safe change to me, since it's so easy to override if you need to.

When programs use calendar-based versioning, I'm left with no information as to whether it's breaking changes or not. In fact, it might as well have no version numbers whatsoever. If I care about backward compatibility, I just have to stick with the exact same unpatched version that I had before. Not a good thing for a language that releases periodic bugfix updates to older versions.

ChrisA

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