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Lessons Learned From A Failed Pull Request

Posted on 6 mins read Tags: Python

Intro: The netrc file

If you already know all about the ~/.netrc file, feel free to skip directly to the next section

But for those who don’t let’s take a closer look at the filename and see if we can find any clues.

  1. It starts with net, so probably something about the network.
  2. It ends with rc, which usually stands for “runtime configuration”.
  3. The path starts with a tilde, which is how UNIX people refer to the home directory. So we are probably not looking at something that is supposed to be used on Windows.
  4. It starts with a dot, which means it’s supposed to be “hidden”. 1
  5. It is a configuration file in the home directory, but not in ~/.config/, which means it does not follow the XDG Base Directory Specification, and thus is probably something a bit old.

I’m not sure how many application rely on the ~/.netrc file. What I do know is that many command-line FTP clients (which is not exactly top notch technology, I know) use this file.

On my latest macOS machine, there’s a ftp binary and I’m still able to use it:

$ ftp
ftp> open mirrors.kernel.org
Trying 2620:3:c000:b::1994:3:14...
Connected to mirrors.pdx.kernel.org.
220 Welcome to mirrors.kernel.org.
Name (mirrors.kernel.org:dmerej): anonymous
331 Please specify the password.
Password:
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp>

Notice how I have to provide a username and password even if I only need anonymous access.

The ~/.netrc file allows me to not have to type my username and password for each connection.

I just need to add a line2 looking like this in the ~/.netrc file:

machine mirrors.kernel.com login dmerej password p4ssw0rd

The bug

So, what does this have to do with Python?

Well, in the Python standard library, (stdlib for short) there’s a module dedicated to parse the ~/.netrc format. The parsing itself is kind of non-trivial. (See the gory details in the GNU documentation)

Here’s how to use it:

import netrc

nrc = netrc.netrc()
login, account, password = nrc.authenticators("example.com")

(The account is an “additional account password”, but I never had to use it)

Anyway, a few months ago I was preparing new machines for doing continuous integration at work.

(We do cross-platform C++ code on Linux, macOS, and Windows)

I decided to use Python for the build scripts, and I needed to store some access token on the nodes.

Since the capabilities granted by the token were quite limited, I did not mind having them in clear text on the nodes themselves, so I decided to use the netrc module.

But on Windows, when running the script directly from cmd.exe, the code crashed with:

Could not find .netrc: $HOME is not set

It’s not that surprising: ~/.netrc is a UNIX thing, so it makes sense it tries using the $HOME environment variable, which on UNIX, is almost always set.

The “fix”

I happen to know3 that in the Python stdlib, there is a function called os.path.expanduser that works very well, even on Windows, and even when HOME is not set.

So I quickly wrote a patch:

  def __init__(self, file=None):
         default_netrc = file is None
         if file is None:
-            try:
-                file = os.path.join(os.environ['HOME'], ".netrc")
-            except KeyError:
-                raise OSError("Could not find .netrc: $HOME is not set") from None
+            file = os.path.join(os.path.expanduser("~"), ".netrc")
         self.hosts = {}
         self.macros = {}
         with open(file) as fp:

and I opened my very first pull request for Python

Trying to get the pull request approved

The process took quite a long time, and the pull request is still not accepted.

For me it was obvious at first that the patch was an improvement. I was re-using existing code, and I did fixed a crash!

But it turned out I had to:

  • Write some tests
  • Patch the documentation
  • Patch the release notes

Let’s be clear, I’m not complaining about this, and I’m not blaming the Python maintainers.

I’ve always cared about testing very much, and I loved how the Python documentation is so accurate, and how the change logs are detailed and precise, and I realize that such requests from the maintainers are required to maintain this level of quality users of the stdlib love so much.

But trying to make the pull request accepted lead me to a new realization:

Sometimes, implementation does not matter.

Implementation does not matter

It turns out that the implementation of the Python stdlib is somewhat special.

That’s because of how this code is used.

People have expectations about it, mostly the fact that the code that uses it is almost always retro-compatible.

The patch I wrote changes the behavior of the stdlib, and for dubious reasons.

If the ~/.netrc file is a UNIX thing, it makes sense that the $HOME variable will have to be set anyway for the rest of the tools to work. (ssh comes to mind).

Also, the netrc() constructor accepts path as parameter, so the “fix” can be written like this:

netrc_path = os.path.expanduser("~/.netrc")
nrc = netrc.netcr(file=netrc_path)

Last but not least, the consequences of the patch are hard to describe and can even lead to breakage.

Consider this code, and assume that ~/.netrc does not exist and $HOME is not set.

try:
    nrc = netrc.netrc()
except OSError as error:
    # do something with error.args

In the old version, error would be a OSError with one argument (the “$HOME is not set” message), but with the new version, the code would instead raise a FileNotFound which is a subclass of OSError, but with two arguments (the errno and the filename)

This could lead to very nasty bugs being introduced.

True, this really is a corner case, but the fact that the change in behavior is so hard to describe is a good indication that maybe it’s not worth it at all.

Going further

I’m not sure what’s next. There are still requests for changes being made, but I’m no longer sure the pull request is a good idea. If you have an opinion on this, I’ll be glad to hear it!

Also, I invite you to watch the Request Under the Hood presentation from PyCon 2017 which is about a similar topic.

Update (2017-11-25)

Well, turned out Berker Peksag, one of Python maintainers, actually cared enough to finish up my pull request and the bug will be fixed for the next Python release. That’s quite a nice happy end.


  1. There’s a nice story by Rob Pike about this topic. [return]
  2. Yes, the password is in clear text, but that’s another topic. [return]
  3. I know it because of this other article I wrote [return]

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