--ignore-installed and dependency resolution

[pradyunsg]

I tried looking for why it was introduced but I couldn't figure out why it was added through the git history. It seems to have been around there since at least pip 0.6.

I'm pretty sure I can implement a Provider to preserve the behavior that it's asking for. I'm actually wondering if this option is in fact relevant once a resolver is added?

[pradyunsg]

/ping @pfmoore @dstufft @xavfernandez

[pfmoore]

Not sure what you're asking. Why does --ignore-installed exist? I don't know the original reason, but I'm pretty sure I've used it to force-install a particular version (without having to first uninstall then reinstall, and no, I don't know why I needed --ignore-installed rather than --force-reinstall). I'm not too worried about having the specific option, but I'd rather we didn't lose the functionality. Of course step 1 is understanding what --ignore-installed does that none of the plethora of other similar options (--force-reinstall, --upgrade-strategy, ...) do. I'm not sure I can help with that 😕

When the new resolver comes in, I think it would be useful to have some use case directed documentation covering how to handle various common scenarios, rather than just a list of options that we leave people to combine as best they can.

[pradyunsg]

Basically, I'm going through "figure out the option mess" phase, so, the situation you describe is exactly what I'm trying to break down and simplify. I guess if nothing else, good documentation sounds like a good idea.

The motivation being that every subtly different behaviour should have a separate class, which means a lot of subtly different behaviours would result in a lot of code. I guess the only way to figure out what to do here is to actually spend time going through everything.

[pfmoore]

Ah, OK. I think --force-reinstall is typically better, as it does a clean uninstall. As I understand it, --ignore-installed is bad because it simply installs over an existing installation, so it won't do a clean uninstall.

So I think the only use cases I see for --ignore-installed is to handle broken installations where metadata is corrupt or something, so that pip can't uninstall an existing version. (It might also be important in the context of --target, where the target directory isn't in site-packages - uninstall metadata might not be visible in that case).

[benoit-pierre]

I don't think there's a valid use case for --ignore-installed, it's way too easy to end up with a mess when using it. Better for users to uninstall manually, then to blindly execute such commands.

[mattficke]

When the new resolver comes in, I think it would be useful to have some use case directed documentation covering how to handle various common scenarios, rather than just a list of options that we leave people to combine as best they can.

Combining ignore-installed with other options (like force-reinstall) can lead to unexpected behavior, additional guidelines about usage would be helpful. Just opened a feature request ticket about disallowing invalid combinations: #9317