Change Python environment default to 3

[ericholscher]

Python 2 is going to stop being maintained in less than 3 years. We should switch our default Python version for projects to Python 3, as this should increase the number of python 3 projects being built on RTD by a large number. It's just a small settings change, but putting it here for discussion.

[ericholscher]

The code to be changed is here: https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/projects/models.py#L200

[Naereen]

Hi,
Is it planned soon to support Python 3.6 by default?
3.7 is already the new default version, but apparently RTD (still) builds with 3.5.
Thanks.

[stsewd]

@Naereen python 3.6 is already supported, you need to use a configuration file https://docs.readthedocs.io/en/latest/yaml-config.html

[Naereen]

Thanks @stsewd !

[humitos]

@ericholscher I think we can do this now. I wonder if we need to create a date Feature flag for this, so only new projects start building with python3 instead of forcing all the projects that were already building with python2 to migrate. What do you think?

[humitos]

Oh, I got confused. By changing the default= attribute in the model we already get what I was proposing with the Feature flag :)

[Naereen]

@Naereen python 3.6 is already supported, you need to use a configuration file https://docs.readthedocs.io/en/latest/yaml-config.html

Hi! I'm quite curious, I though python 3.6 was supported, but for instance this build tried to use python.version: 3.6 in my YAML file, and failed instantly.
See #4918

[stsewd]

It's supported, please see https://docs.readthedocs.io/en/latest/yaml-config.html#python-version

The supported Python versions depends on the version of the build image your project is using.

You need

build:
  image: latest

[Naereen]

Oh, OK. Sorry I (still) didn't understand this point. Thanks!
My latest build is working, thanks a lot!

[stsewd]

We use docker images to encapsulate the build process, each docker image has different python versions, and libs, here is the list https://docs.readthedocs.io/en/latest/builds.html#the-build-environment. Using latest is recommendable.

[impredicative]

Is there any reason why 3.6 isn't currently the default?

[humitos]

Is there any reason why 3.6 isn't currently the default?

Just legacy code. We are upgrading it to make this to happen soon.