Disallow using both the --ignore-installed and --force-reinstall options at the same time

[mattficke]

What's the problem this feature will solve?

When running pip install with both the --ignore-installed and --force-reinstall flags, --force-reinstall is effectively disabled by --ignore-installed. It's not entirely intuitive that these options are in conflict and that ignore-installed has priority.

Here's my understanding of what happens. The force-reinstall option only takes effect if the package being installed is returned by the get_dist_to_uninstall function:

pip/src/pip/_internal/resolution/resolvelib/resolver.py

Lines 135 to 143 in 89d50dd

	# Check if there is already an installation under the same name,
	# and set a flag for later stages to uninstall it, if needed.
	installed_dist = self.factory.get_dist_to_uninstall(candidate)
	if installed_dist is None:
	# There is no existing installation -- nothing to uninstall.
	ireq.should_reinstall = False
	elif self.factory.force_reinstall:
	# The --force-reinstall flag is set -- reinstall.
	ireq.should_reinstall = True

get_dist_to_uninstall checks the _installed_dists dict to see if the package is already installed:

pip/src/pip/_internal/resolution/resolvelib/factory.py

Lines 323 to 328 in 89d50dd

	def get_dist_to_uninstall(self, candidate):
	# type: (Candidate) -> Optional[Distribution]
	# TODO: Are there more cases this needs to return True? Editable?
	dist = self._installed_dists.get(candidate.name)
	if dist is None: # Not installed, no uninstallation required.
	return None

Before any of this happened, though, _installed_dists was set to an empty dict by the ignore-installed option:

pip/src/pip/_internal/resolution/resolvelib/factory.py

Lines 102 to 108 in 89d50dd

	if not ignore_installed:
	self._installed_dists = {
	canonicalize_name(dist.project_name): dist
	for dist in get_installed_distributions(local_only=False)
	}
	else:
	self._installed_dists = {}

This prevents force-reinstall from having any effect.

Describe the solution you'd like

This behavior seems to mean that there's never any situation in which you'd want to (or be able to) use --ignore-installed and --force-reinstall at the same time. Given this, can pip throw an error if both flags are used? Explicitly failing with instructions to choose one seems better than silently disabling force-reinstall.

Say you're trying to resolve some mysterious dependency problems. You're using the force-reinstall option, but you're still having issues, so you look at the available pip install options and see ignore-installed. You might try that too, thinking that the effect will be additive, but this silently disables force-reinstall instead, sending you on a wild goose chase.

Alternative Solutions

A warning could be printed when using both flags, instead of throwing an error. The ignore-installed help text could also be updated to inform users that it shouldn't be used with the force-reinstall option.

I think throwing an error would be better since this seems like a fundamentally invalid combination of options, but warning messages or usage instructions would be less intrusive.