This post is about the thing in the title.

I used to work for Yelp. For historical reasons — probably “the initial developers preferred it” — their mostly-Python codebase had always been indented with tabs. That’s in stark contrast to the vast majority of the Python ecosystem, which generally uses the standard library’s style guide recommendation of four spaces. The presence of tabs caused occasional minor headaches and grumbles among the Python developers, who now numbered in the dozens and were generally used to spaces.

At the end of 2013, I bestowed Yelp with a Christmas gift: I converted their entire primary codebase from tabs to four spaces. On the off chance anyone else ever wants to do the same, here’s how I did it. Probably. I mean, it’s been two and a half years, but I wrote most of this at the time, so it should be correct.

Please note: I do not care what you think about tabs versus spaces. That’s for a different post! I no longer work for Yelp, anyway — so as compelling as your argument may be, I can no longer undo what I have done.

Prerequisites

First, be absolutely sure you’re never going to change your mind. If you as an organization are ambivalent about your whitespace needs, or if you have influential coworkers who will use this post for evil to switch everything back to tabs as soon as you leave, you may wish to reconsider.

Fix mixed indentation

If you’re using a whitespace-sensitive language, you must fix any inconsistent indentation. (You might want to do this anyway, or your code will look like nonsense.) By “inconsistent”, I mean any code that will change relative indentation levels if the width of a tab changes. Consider:

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....if foo:
------->print("true")

If a tab is considered to be eight cells wide, as the good Lord intended, this is fine. But if you’re about to replace all your tabs with four spaces, the extra indentation level vanishes, and suddenly this is invalid Python.

You would not believe how many cases of this I found. I most fondly remember the file that was for some reason indented with n tabs plus a single space, except where it wasn’t. I have no idea how that happened. (Incidentally, the need to micromanage variable-width invisible characters is one of the reasons I wanted to get rid of tabs.) (Please don’t leave comments about this.) (Also consider set shiftround if you use vim, which pretty well eliminates this problem but is tragically under-used.)

On the other hand, if you are some kind of monster and want to replace every tab with eight spaces, then you don’t need to worry about this.

You could just scan your codebase for leading spaces, but if you have a mix of tabbed files and spaced files, you’ll get a ton of false positives and it’ll be a huge pain in the ass. A somewhat more robust approach for Python specifically is:

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python -tt -m compileall . | grep Sorry

-tt tells the interpreter to treat inconsistent indentation as a SyntaxError. The compileall module searches recursively for .py files and produces .pyc bytecode, which requires parsing each file, which will trigger the SyntaxError. And any errors encountered while compiling modules produce a line starting with Sorry, along with the filename, line number, and column number.

Now you can spend an afternoon fixing those all by hand and trying to figure out why this one file seems to have been written with a tabstop of 3.

Distribute a Git filter definition

The actual process uses a Git filter to enforce that no new tabs find their way into the repository and fix any tabs on in-flight branches. The configuration for which filters to run on which files is stored as part of the repository, but unfortunately, the configuration for what each filter does is not.

One way or another, you must get this block in your devs’ Git configuration — anyone doing regular development who doesn’t have the filter definition will be utterly confused. This is probably the hardest part of the process. Thankfully, Yelp mostly does work on beefy shared dev machines, so I only had to bug an ops person to stick this incantation in /etc/gitconfig and wait for Puppet. YMMV.

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[filter "spabs"]
    clean = expand --initial -t 4
    smudge = expand --initial -t 4
    required
[merge]
    renormalize = true

I’ll explain what this all actually does later. Oh, and it might help to actually have expand installed. Most Unix-likes should have it already; if you have developers using Windows, expand is one of the utilities in the unixutils project. Also, BSD (i.e. OS X) expand apparently doesn’t have the --initial argument, but unless you’re in the habit of sprinkling tab characters inside string literals, you can safely leave it off.

Doing it

Here’s the good part.

If at all possible, get all your collaborators to stop doing work for a day while you get this sorted. Christmas works pretty well! I did this on December 26, when we were so short-staffed that there weren’t even any deploys scheduled.

Invoking Git's nuclear option

First, create or amend .gitattributes in the root of your repository with the following:

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*.py    filter=spabs

You can add as many source-like filetypes as you want by adding more lines with different extensions. I converted everything I could find that we’d used in any repository, including but not limited to: .css, .scss, .js, .html, .xml, .xsl, .txt, .md, .sh, etc. (I left .c and .h alone. It seemed somehow inappropriate to change tabbed C code.)

Here’s a brief explanation. .gitattributes is a magical file that tells Git how to handle the contents of files. The most common use is probably line-ending conversion behavior for projects edited on both Windows and Unix; I’ve also seen some use of it to define language-aware diffing for certain files (i.e. for each hunk, figure out what function it lives in, and put the function name in the hunk’s header line).

What I’ve done here is add a custom filter, i.e. a program run on checkin and checkout. The actual program, expand, was listed in the Git configuration you (hopefully) distributed to everyone. When Git sticks a file in the repository (via add, commit, whatever), it runs the clean filter; when it updates a file on disk based on the repository, it runs the smudge filter. In this case I want to be extra sure there are never any tabs anywhere, so I made both filters do the same thing: convert all leading tabs to four spaces. (The required line from the config will cause Git to complain if expand doesn’t exit with 0 — that means something has gone really wrong.)

This isn’t perfect, as we’ll see later, but it’s some gentle protection against letting tabs sneak into your codebase. I hope we can all agree that mixing tabbed lines with spaced lines is far worse than either tabs or spaces.

If you like, you can commit .gitattributes separately. If you do, DO NOT PUSH YET.

Performing the conversion

I’m paranoid, and Yelp’s codebase was colossal, so I wrote a whole script that inspected every single text-like file in the codebase and manually ran expand on it and very carefully and idempotently adjusted .gitattributes. The nice thing about this was that anyone else could then run the script against one of Yelp’s myriad smaller repositories, without having to understand any of this Git witchcraft. (Gitchcraft?) Unfortunately, I quit and don’t have it any more.

The much faster way to do this is:

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git checkout HEAD -- "$(git rev-parse --show-toplevel)"

This asks git checkout to re-checkout every single file in your whole repository. As a side effect, the smudge command will be run, converting all your tabs to spaces. You will end up with a whole lot of whitespace changes.

You may want to run your test suite right about now.

Then, commit! As per Yelp tradition when rewriting every single file in the whole codebase, I attributed the commit to Yelp’s lovable mascot Darwin. It stands out better in git blame, and it preserved the extremely critical integrity of my commit stats.

Push to master and you’re done. More or less.

Effects on Git

I think somewhere in the neighborhood of two million lines were affected, and Git handled it surprisingly well. The commit was basically instant, and there weren’t any noticeable performance problems going forward, save for a couple minor wrinkles explained later.

The impact on Git workflow is fairly minimal. Most of the complications will happen to people who are performing casual wizardry anyway and thus probably know a little bit about what they’re doing. Developers who just commit and merge shouldn’t have any problems.

• A fresh checkout of the repository (or master, at least) will contain spaces, of course, because the checked-in files contain spaces.

• Any inflight branches will contain tabs, because they haven’t seen the .gitattributes file or the mass conversion commit.

• Merging an inflight branch with master (in either direction) will transparently convert all the tabs on the branch to spaces before merging. The developer shouldn’t even notice that anything special happened at all.

  This is the magical thing the merge.renormalize setting does. renormalize is an option for the default merge strategy (recursive) that applies filters before performing the merge; the merge.renormalize setting turns it on by default for git merge. Since the merged .gitattributes contains the filter, it gets applied to both sides. I think.

  Note: I don’t know if renormalize works with more exotic merge strategies. I also don’t know what happens if there are merge conflicts within .gitattributes itself.

  Note 2: renormalize does not apply to new files created in the branch — they only exist on one side, so there’s no need to merge them. See below.

• Rebasing an inflight branch will not work, or more precisely will produce a zillion merge conflicts. merge.renormalize doesn’t apply to git rebase, and there is no rebase.renormalize setting.

  Luckily, you can do the same thing manually with -X. git rebase -Xrenormalize origin/master should work fine.

  -X is supported by all Git commands that do anything resembling a merge, so the same applies to e.g. git cherry-pick or git pull --rebase. You can use it with git merge, as well, but the setting makes it unnecessary.

• Old stashes probably won’t apply cleanly, and git stash apply tragically ignores -X. I know of two workarounds:

  1. Convert the stash to a branch with git stash branch, then merge or rebase or whatever as above.

  2. Apply the stash manually with, e.g., git cherry-pick 'stash@{0}' -n -m 1 -Xrenormalize. You need the -m 1 (“use diff against parent 1”) because under the hood, a stash is a merge between several distinct commits that hold different parts of the stash, and cherry-pick needs to know which parent to diff against to create a patch. -n just prevents committing, so your stash description of “wip: this doesn’t fucking work” isn’t automatically turned into a commit message.

• Blame is not, in fact, permanently ruined. git blame -w ignores whitespace-only changes.

• The total size of your repository will increase, but not by nearly as much as you’d think. Git ultimately stores compressed binary patches, and a patch that contains mostly the same two characters compresses really well. I want to say Yelp’s repository only grew by 1% or so. (The increase may be larger short-term, but git gc will eventually compress it all away.)

Possible fallout

Relatively minor, considering the magnitude of the change. Some short-term, some persisting for the life of your project, sorry.

Old branches that introduced new tabbed files

About a week after the conversion, as developers trickled back from being on vacation, there was a sudden surge of confusion about a phantom file listed in git status. It would be marked as modified, and no amount of git checkout or git reset would make it go away. Everyone with this problem saw the same file marked as modified, but nobody had touched it.

It turned out that someone had had an inflight branch with a newly-created file, indented with tabs. This branch had been merged into master about a week after the conversion, and developers were seeing the new file show up as phantom-modified after their next interactions with master.

The problem was that git dutifully applied the smudge filter when checking this file out, converting it to spaces on-disk… but the copy in the repository still had tabs, making it appear modified. git checkout didn’t fix this because it had caused the problem in the first place: a checkout would again run the filter and produce a modified file. (I suspect this wouldn’t have happened if our clean and smudge had actually been inverses and the repository had remained tabbed, but we explicitly didn’t want that.)

Fixing this was simple enough: I told everyone to just commit the phantom changes in a separate commit whenever this happened. (If the file had also been modified, git diff -w would show a “clean” diff.) The whitespace change would happen in multiple commits, but they’d all merge cleanly as they hit master, since they all contained the same change. Once the checked-in copy of the file contained spaces, the problem disappeared.

I saw a few instances of this over the first few weeks, but they all sorted themselves out as devs committed Git’s whitespace change. I think it could’ve been prevented with a clever git hook that applies filters to new files during a merge, but that would’ve been much more complicated.

Intermittently slow git status

One or two developers saw git status be preposterously slow, taking a minute or more, rather than less than half a second.

Some strace revealed that expand was being run tens of thousands of times. Whoops!

The developers who ran into this ended up making a fresh clone, which mysteriously resolved the issue. My best guess is that we were accidentally hitting the slow path in git status—the solution to the “racy Git” problem. I had to do some guessing here, because the implications aren’t fully described in that documentation, and very few people seem to have ever run into this.

Essentially, Git cheats a bit to keep “is this file changed?” fast: it compares only file stats, like size and mtime. Git has a file called the “index”, which contains, well, the index: it’s a description of what the next commit will look like if you run a plain git commit. The index also remembers which files on disk were modified, as of the last time it was written. So if a file’s mtime is older than the index’s mtime, it’s safe to assume the index is still correct. But it’s also possible that a file was changed in a way that kept its size the same immediately after the index was last written — so quickly that the mtimes for both are identical.

To fix this, if Git sees a file that it thinks is unmodified, and the mtime is exactly the same (or newer, of course) than the index’s mtime, Git will compare the file’s full contents to whatever’s in the index. Naturally, this takes much longer than just stating a bunch of files.

Now imagine someone switches from a very old branch to master. A great many files are updated in the process, but the machine is fast enough that all the updated files and Git’s index end up with the same mtime.

I only discovered this explanation after all the affected developers had given up and recloned, so I never found out for sure whether it was the true case, and I never saw it happen again. But that seems plausible.

If you find yourself with an index with a slow cache, you just need to do something that updates the index. Read-only commands like git status or git diff won’t do it, but git add will. If you really don’t have anything to add yet, you can force an update manually:

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git update-index somefile

somefile can be any arbitrary file that’s in the repository. This command forces Git to examine it and write its modifiedness to the index—as a side effect, the index will now be updated.

Final cleanup

Once everything has settled, you may want to remove all the filter stuff and just add a pre-commit hook that rejects tabs outright.

You can also tell your developers that they can finally remove all their .vimrc hacks for switching to tabs specifically in your codebase. (Maybe tell them they should’ve been using vim-sleuth.)