Git: Merge one folder inside a repo

Question

I have an unusual need, and I'm wondering whether Git could fill it.

I want to port my Python package, python_toolbox to Python 3. But I don't like the idea of using 2to3, nor supporting both Python 2 and Python 3 using the same code. (Because it's important for me that my code will be beautiful, and I don't find code written for both Python 2 and Python 3 to be beautiful.)

What I want is to have 2 separate source folders, one for Python 2.x and one for Python 3.x. This will allow me to write each version of the code tailored to the respective major Python version. I want both folders to be in the same repo, and setup.py will choose between them dynamically depending on the version of Python running it. So far so good.

Now, here is where I need help: I want to be able to do merges from my Python 2.x source folder to my Python 3.x source folder. Why? When I develop a feature on the Python 2.x folder, I want have those feature on the Python 3.x version too. I don't want to copy them manually. I want to merge them into the Python 3.x folder, and I fully expect to have wonderful merge fails where I'll have to use my judgement to decide how to merge features that were implemented for Python 2.x into code that was modified for Python 3.x.

The question is: How can I do that? Those folders are folders inside a Git repo, they're not Git repos themselves. I thought about using Git submodules, which I've never used before, but reading about them online paints a scary picture. (The term "sobmodules" had been thrown around.)

Any other ideas how I could merge between these folders in my Git repo?

Answer 1

I recommend you using branches. Dedicate your branches to either version. You may use git branch --orphan to create a fully independent branch. (That may make merging harder, as git wont't be able to find a common ancestor.)

Anyway, if you go with that solution you will be able to merge from one version into another. You will also be able to clone both versions in one command (as they are in the same repo). However, to able to have both versions open at the same time, you will need to have the repo cloned two times so you can have two different branches checked out at the same time.

Answer 2

You could create branches by having the two versions in separate repositories and use the other one as a remote. The toplevel dir with setup.py and any PyPi meta information, readme's, etc, would also be a repository. The directory layout would look like this:

/root/
   .git/
   setup.py
   read.me
   python2/
      .git/
      source.py
   python3/
      .git/
      source.py

The two sub repositories can be linked so that you can merge between them with e.g.

cd /root/python2
git remote add python3 ../python3
cd /root/python3
git remote add python2 ../python2

Then you can do the usual git fetch, cherry-pick, or even merge between them.

In the main repo, and for releasing things, you use the git submodules feature to coordinate which version of the individual sub repositories you'd like to have checkedout to have a consistent view of the project.

There's lots of stuff in the internet on git's submodules. I'd start with this question on nested repos and work your way through the links and docs.

Here's an explanation of subtree merges and compares it to working with submodules. Basically, subtree merges would combine the idea of having ordinary branches for Py2 and Py3 (like in the answer by Oznerol256) in one repo, with the idea of having a hierarchically organized repo.