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

[pdegroote]

What's the problem this feature will solve?

When installing packages in development mode, and sloppy removal of those folders (instead of uninstall), the error

Egg-link {} does not match installed location of {} (at {}) (pip/req_uninstall.py, line 535)

is sometimes raised when installing a new package of the same name from a different folder. The issue can be solved by manually removing the egg-link, but it may sometimes be difficult to find it, certainly for people not very experienced with pip (it could be installed in site-packages of the python installation, or maybe UserData on Windows, ...). A better error message should make it easier to find it, and remove it.

Describe the solution you'd like

I suggest to change this code (starting line 529 in pip/req_uninstall)

elif develop_egg_link:
            # develop egg
            with open(develop_egg_link) as fh:
                link_pointer = os.path.normcase(fh.readline().strip())
            assert (
                link_pointer == dist.location
            ), "Egg-link {} does not match installed location of {} (at {})".format(
                link_pointer, dist.project_name, dist.location
            )

with this (adding not only the contents of "develop_egg_link" to the error message, but also the path of the file itself):

elif develop_egg_link:
            # develop egg
            with open(develop_egg_link) as fh:
                link_pointer = os.path.normcase(fh.readline().strip())
            assert (
                link_pointer == dist.location
            ), "Egg-link {} at {} does not match installed location of {} (at {})".format(
                link_pointer, develop_egg_link, dist.project_name, dist.location
            )

Alternative Solutions

I don't know about any alternative solutions; maybe forcing uninstall could help eventually, but if this happens in the course of a CI pipeline, then an improved error message is saves a lot of time.

Additional context

I've never contributed to pip, but wouldn't mind making this contribution myself, it doesn't seem to pose any big risks.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

@sbidoul Do your changes around PEP 610 touch this part of the source?

[sbidoul]

@uranusjr #10249 should not change this part, as the refactoring is meant to preserve the behaviour.

@pdegroote your proposal sounds reasonable and safe enough. Feel free to open a PR. Could you also provide steps that trigger the assert ?