Testing a GitHub Pull Request (Shallow Thoughts)

Testing a GitHub Pull Request

Several times recently I've come across someone with a useful fix to a program on GitHub, for which they'd filed a GitHub pull request.

The problem is that GitHub doesn't give you any link on the pull request to let you download the code in that pull request. You can get a list of the checkins inside it, or a list of the changed files so you can view the differences graphically. But if you want the code on your own computer, so you can test it, or use your own editors and diff tools to inspect it, it's not obvious how. That this is a problem is easily seen with a web search for something like download github pull request -- there are huge numbers of people asking how, and most of the answers are vague unclear.

That's a shame, because it turns out it's easy to pull a pull request. You can fetch it directly with git into a new branch as long as you have the pull request ID. That's the ID shown on the GitHub pull request page:

Once you have the pull request ID, choose a new name for your branch, then fetch it:

git fetch origin pull/PULL-REQUEST_ID/head:NEW-BRANCH-NAME
git checkout NEW-BRANCH-NAME

Then you can view diffs with something like git difftool NEW-BRANCH-NAME..master

Easy! GitHub should give a hint of that on its pull request pages.

Fetching a Pull Request diff to apply it to another tree

But shortly after I learned how to apply a pull request, I had a related but different problem in another project. There was a pull request for an older repository, but the part it applied to had since been split off into a separate project. (It was an old pull request that had fallen through the cracks, and as a new developer on the project, I wanted to see if I could help test it in the new repository.)

You can't pull a pull request that's for a whole different repository. But what you can do is go to the pull request's page on GitHub. There are 3 tabs: Conversation, Commits, and Files changed. Click on Files changed to see the diffs visually.

That works if the changes are small and only affect a few files (which fortunately was the case this time). It's not so great if there are a lot of changes or a lot of files affected. I couldn't find any "Raw" or "download" button that would give me a diff I could actually apply. You can select all and then paste the diffs into a local file, but you have to do that separately for each file affected. It might be, if you have a lot of files, that the best solution is to check out the original repo, apply the pull request, generate a diff locally with git diff, then apply that diff to the new repo. Rather circuitous. But with any luck that situation won't arise very often.

Update: thanks very much to Houz for the solution! (In the comments, below.) Just append .diff or .patch to the pull request URL, e.g. https://github.com/OWNER/REPO/pull/REQUEST-ID.diff which you can view in a browser or fetch with wget or curl.