pkgsrc lists these features of pip, emphasis added:

  • All packages are downloaded before installation. Partially-completed installation doesn’t occur as a result.
  • Care is taken to present useful output on the console.
  • The reasons for actions are kept track of. For instance, if a package is being installed, pip keeps track of why that package was required.
  • Error messages should be useful.
  • The code is relatively concise and cohesive, making it easier to use programmatically.
  • Packages don’t have to be installed as egg archives, they can be installed flat (while keeping the egg metadata).
  • Native support for other version control systems (Git, Mercurial and Bazaar)
  • Uninstallation of packages.
  • Simple to define fixed sets of requirements and reliably reproduce a set of packages.

I’d say the emphasised lines would be useful for any tool, not least a package manager.