I've had this cooking in my head for a bit now. These aren't critical path for some feature but this definitely something I want to do to make pip more approachable for new contributors and make it easier to reason about the installation flow. I've hinted this before in #4713 (comment).

The idea is that, eventually, the resolver should consume Requirements, get Candidates during resolution and spew out Distributions. The Distributions would then be operated upon during installation/uninstallation etc, downloading/caching would deal with Candidates, listing/freezing/checking would deal with Distributions. This 3-class model works cleanly for each of these use cases.

Currently, all three "jobs" are served by InstallRequirement (and InstallationCandidate :P), and it's sometimes hard to keep track of what's happening since there's a lot of stuff that's happening in that one class.

This would result in:

• sanity while actually keeping track of the installation flow
• make it easier later on to improve unit test coverage - scope for improving test speed
• better decoupling of UI elements from actual code -- easier Redesigning pip's output (for install, wheel and download commands) #4649
• ease when modifying behaviours for certain kinds of Distributions -- easier to implement PEP 517/518

I'm curious what @pypa/pip-committers and others think about this since, all in all, this work would result in changes in about a quarter of pip._internal, in terms of LOC.

What follows is basically the mind-dump for this (from about 4 months back, I think):

• Refactor Refactor Refactor! {confirm it looks fine, figure out path}
  • Do away with all the respect for InstallRequirement and start chopping it.
  • Rename InstallationCandidate to CandidateDistribution
  • Move all "link" related information into CandidateDistribution
  • Create pip._internal.models.requirement.Requirement - Holds information that is given by the user
    • Essentially, a helpful wrapper around packaging.requirements.Requirement
  • pip._internal.models.candidate.CandidateDistribution - Holds information fetched from the index
    • package-version visible to the resolver
    • url-hash visible to download logic
    • TODO: check how is this different from current InstallationCandidate
    • TODO: Figure out how VCS looks with this?
  • pip._internal.models.distribution.Distribution - Describing the build, install, uninstall and other related behaviours for different kinds of distributions.
    • might wrap a pkg_resources.Distribution?
    • LegacySourceDistribution (non PEP 518/517)
    • ModernSourceDistribution (PEP 518/517)
    • WheelDistribution
    • Maybe do this early?
  • An intermediator to hold the logic of conversion from CandidateDistribution to a Distribution object
    • need to "build" to get dependencies and Distribution should hold that behaviour, not CandidateDistribution.

The reason I want to use the pip._internal.models is to represent that these are essentially objects representing data stored elsewhere that would be operated upon and isn't that what models are, in some sense? :P

PS: This is after I am done with zazo and then bringing it in. zazo is the project where I'm making the resolver.