Expand pip's install-time conflict checking to the wheel and download commands

[pradyunsg]

What's the problem this feature will solve?
pip install today warns when it installs an incompatible set of packages. pip wheel and pip download do not.

Describe the solution you'd like

Expanding the warnings to also be shown when running pip wheel or pip download would be helpful as the same information is relevant even then.

See #988 (comment).

Alternative Solutions

None explored.

Additional context

N/A

[pfmoore]

Not sure I follow here. pip wheel and pip download don't install anything, so what conflicts are involved?

Suppose I do pip wheel A B where A depends on B<2.0 and B 2.0 exists. This will download A and B 2.0. But I could already have B 1.0 in my wheelhouse, or I could download it later. If I do, then pip install A will still work from that wheelhouse. So what's the conflict? It's not as if pip wheel B will delete older versions of B when downloading B 2.0.

It seems to me that this might help one particular use case, at the cost of potentially producing confusing (and arguably incorrect) warnings for other use cases. I'd like to see a clearer justification, as well as a discussion of how to not trigger incorrect warnings in other cases (I'm not a fan of the "it's just a warning so you can ignore it if you know it doesn't matter" argument).

[pradyunsg]

My initial thought here was that pip does end up downloading the incorrect versions of the packages (much like it would for pip install). I just wanted to file it somewhere until I have some time later to flesh out the thought.

[pfmoore]

Agreed, it's not impossible that we could do something helpful here. I'm just flagging up that because the target directory can contain multiple versions (unlike install), what counts as "downloading the incorrect versions of the packages" is not necessarily something we can assume is the same as for install. And in particular, the way I use pip wheel, the OP's use case wouldn't be "incorrect" for me.

No need for further debate on the matter until there's a PR, though.

[notatallshaw]

I think this is no longer relevant? pip download never downloads conflicting packages.