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"""A provider that handles packages with "extras". Python package dependencies can include "extras", which are additional dependencies that are installed "on demand". For instance, project X could have an additional set of dependencies if PDF generation features are needed. These can be defined for an extra "pdf" and requested on install as X[pdf]. The basic resolvelib algorithm cannot handle extras, as it builds a dependency graph which needs to be static - the edges (dependencies) from a node (candidate) must be fixed. Extras break this assumption. To model projects with extras, we define a candidate as being a project with a specific set of dependencies. This introduces a problem, as the resolver could produce a solution that demands version 1.0 of X[foo] and version 2.0 of X[bar]. This is impossible, as there is actually only one project X to be installed. To address this, we inject an additional dependency for every candidate with an extra - X[foo] version v depends on X version v. By doing this, we constrain the solution to require a unique version of X. """ from resolvelib.providers import AbstractProvider class ExtrasProvider(AbstractProvider): """A provider that handles extras.""" def get_extras_for(self, requirement_or_candidate): """Given a requirement or candidate, return its extras. The extras should be a hashable value. """ raise NotImplementedError def get_base_requirement(self, candidate): """Given a candidate, return a requirement that specifies that project/version. """ raise NotImplementedError def identify(self, requirement_or_candidate): base = super(ExtrasProvider, self).identify(requirement_or_candidate) extras = self.get_extras_for(requirement_or_candidate) if extras: return (base, extras) else: return base def get_dependencies(self, candidate): deps = super(ExtrasProvider, self).get_dependencies(candidate) if candidate.extras: req = self.get_base_requirement(candidate) deps.append(req) return deps