Improve error message when egg-link does not match installed location

[pdegroote]

Improve error message when a pip package cannot be reinstalled after an egg link to a development package persists, but the original folder has been deleted.

This may help in finding the location of the egg link that needs to be manually removed by a user to solve the problem. It appears it only occurs in very specific situations, and maybe even only before PEP517. I encountered the issue in my company on our build server, but it wasn't trivial to reproduce it locally (I took an older version of Python and legacy pip option just to be sure, and I also couldn't have a toml file in the main directory). The following workflow reproduces the issue (but I'm not 100% sure what are the required steps, maybe some of the steps below allow variations). I tested it only on Windows 10.

1. Install Python 3.8.10 in an administrator folder (e.g. C:\Program Files\Python)
2. Clone the pypa sample project to a directory, e.g. "sample1", and remove the toml and tox.ini file
3. Install the sample project in a user directory in development mode:
   "C:\Program Files\Python\python.exe -m pip install --user --no-use-pep517 -e .
4. Delete sample1 (or rename, to invalidate the directory)
5. Clone the pypa sample project to a new directory (e.g. "sample2" and remove the toml and tox.ini file
6. Try to install sample2 as user "C:\Program Files\Python\python.exe -m pip install --user --no-use-pep517 -e .

I get the following error message:

The problem is that it is not clear where the conflicting egg link is located, which is particularly difficult when working from build accounts where user directories may not be directly clear.

For a description of the issue, see #10476 ( #10476. )

[pradyunsg]

FWIW, I have a local change that's a part of #10421 that'll conflict with this one -- it is one of the follow ups I have for when #10538 is merged, since going down that PR's framework makes it much nicer to be able to present better error messages.

[uranusjr]

Now this needs a news file and some fixes to satisfy checkers.

[uranusjr]

I think this is worth listing as a feature, not trivial.

[pradyunsg]

Hmm... @pdegroote would it be OK if I pick this PR up and make CI be happy with it?

[pdegroote]

Hmm... @pdegroote would it be OK if I pick this PR up and make CI be happy with it?

Yes please! This is my first attempt at contributing to pip, and still need to learn the flow.

[pradyunsg]

Yes please! This is my first attempt at contributing to pip, and still need to learn the flow.

Sure, happy to help!

Here's what I've done to update this PR:

• I ran git rebase --interactive main (here, main refers to the main branch in my local clone, not your PR -- see https://hackernoon.com/beginners-guide-to-interactive-rebasing-346a3f9c3a6d which seems like a reasonable guide to this).
  • Squashed all the code changes into a single commit, and newfile changes into a second commit.
• Pushed and waited for CI to run -- it failed with a pre-commit issue and a docs issue.
  • the "docs" build is also failing in the main branch of pip in the same way, which means the failure is unrelated to this change.
    • (Fix our documentation build and changelog rendering #10607 is a separate change to fix that)
  • the "pre-commit" build says that some content isn't properly formatted.
    • I ran nox -s lint (you can also run tox -e lint) locally, to run the automated code formatters and code style checks and made sure there's no other errors.
• Made a commit fixing the reported failures.
• Did another interactive rebase, squashing the above fixes into the first commit (the one that did the code changes).
  • Waited, and saw that pre-commit is happy + docs is as it is in pip's main branch. :)

---

One suggestion: Make a new branch (like git branch improve-egg-link-error-message followed by git checkout improve-egg-link-error-message).

• It is generally a "best practice" to not make a pull request from your fork's main branch.
• Ideally, your fork's main branch reflects the same contents as the main branch on this repository.

[DiddiLeija]

Hi @pdegroote. This PR looks good!

Would you like to rebase this PR, or do you prefer to give us the chance to do it?