text: import contextlib
  import functools
  import logging
  from typing import (
      TYPE_CHECKING,
      Dict,
      FrozenSet,
      Iterable,
      Iterator,
      List,
      Mapping,
      NamedTuple,
      Optional,
      Sequence,
      Set,
      Tuple,
      TypeVar,
      cast,
  )
  
  from pip._vendor.packaging.requirements import InvalidRequirement
  from pip._vendor.packaging.requirements import Requirement as PackagingRequirement
  from pip._vendor.packaging.specifiers import SpecifierSet
  from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
  from pip._vendor.resolvelib import ResolutionImpossible
  
  from pip._internal.cache import CacheEntry, WheelCache
  from pip._internal.exceptions import (
      DistributionNotFound,
      InstallationError,
      InstallationSubprocessError,
      MetadataInconsistent,
      UnsupportedPythonVersion,
      UnsupportedWheel,
  )
  from pip._internal.index.package_finder import PackageFinder
  from pip._internal.metadata import BaseDistribution, get_default_environment
  from pip._internal.models.link import Link
  from pip._internal.models.wheel import Wheel
  from pip._internal.operations.prepare import RequirementPreparer
  from pip._internal.req.constructors import install_req_from_link_and_ireq
  from pip._internal.req.req_install import (
      InstallRequirement,
      check_invalid_constraint_type,
  )
  from pip._internal.resolution.base import InstallRequirementProvider
  from pip._internal.utils.compatibility_tags import get_supported
  from pip._internal.utils.hashes import Hashes
  from pip._internal.utils.virtualenv import running_under_virtualenv
  
  from .base import Candidate, CandidateVersion, Constraint, Requirement
  from .candidates import (
      AlreadyInstalledCandidate,
      BaseCandidate,
      EditableCandidate,
      ExtrasCandidate,
      LinkCandidate,
      RequiresPythonCandidate,
      as_base_candidate,
  )
  from .found_candidates import FoundCandidates, IndexCandidateInfo
  from .requirements import (
      ExplicitRequirement,
      RequiresPythonRequirement,
      SpecifierRequirement,
      UnsatisfiableRequirement,
  )
  
  if TYPE_CHECKING:
      from typing import Protocol
  
      class ConflictCause(Protocol):
          requirement: RequiresPythonRequirement
          parent: Candidate
  
  
  logger = logging.getLogger(__name__)
  
  C = TypeVar("C")
  Cache = Dict[Link, C]
  
  
  class CollectedRootRequirements(NamedTuple):
      requirements: List[Requirement]
      constraints: Dict[str, Constraint]
      user_requested: Dict[str, int]
  
  
  class Factory:
      def __init__(
          self,
          finder: PackageFinder,
          preparer: RequirementPreparer,
          make_install_req: InstallRequirementProvider,
          wheel_cache: Optional[WheelCache],
          use_user_site: bool,
          force_reinstall: bool,
          ignore_installed: bool,
          ignore_requires_python: bool,
          py_version_info: Optional[Tuple[int, ...]] = None,
      ) -> None:
          self._finder = finder
          self.preparer = preparer
          self._wheel_cache = wheel_cache
          self._python_candidate = RequiresPythonCandidate(py_version_info)
          self._make_install_req_from_spec = make_install_req
          self._use_user_site = use_user_site
          self._force_reinstall = force_reinstall
          self._ignore_requires_python = ignore_requires_python
  
          self._build_failures: Cache[InstallationError] = {}
          self._link_candidate_cache: Cache[LinkCandidate] = {}
          self._editable_candidate_cache: Cache[EditableCandidate] = {}
          self._installed_candidate_cache: Dict[str, AlreadyInstalledCandidate] = {}
          self._extras_candidate_cache: Dict[
              Tuple[int, FrozenSet[str]], ExtrasCandidate
          ] = {}
  
          if not ignore_installed:
              env = get_default_environment()
              self._installed_dists = {
                  dist.canonical_name: dist
                  for dist in env.iter_installed_distributions(local_only=False)
              }
          else:
              self._installed_dists = {}
  
      @property
      def force_reinstall(self) -> bool:
          return self._force_reinstall
  
      def _fail_if_link_is_unsupported_wheel(self, link: Link) -> None:
          if not link.is_wheel:
              return
          wheel = Wheel(link.filename)
          if wheel.supported(self._finder.target_python.get_tags()):
              return
          msg = f"{link.filename} is not a supported wheel on this platform."
          raise UnsupportedWheel(msg)
  
      def _make_extras_candidate(
          self, base: BaseCandidate, extras: FrozenSet[str]
      ) -> ExtrasCandidate:
          cache_key = (id(base), extras)
          try:
              candidate = self._extras_candidate_cache[cache_key]
          except KeyError:
              candidate = ExtrasCandidate(base, extras)
              self._extras_candidate_cache[cache_key] = candidate
          return candidate
  
      def _make_candidate_from_dist(
          self,
          dist: BaseDistribution,
          extras: FrozenSet[str],
          template: InstallRequirement,
      ) -> Candidate:
          try:
              base = self._installed_candidate_cache[dist.canonical_name]
          except KeyError:
              from pip._internal.metadata.pkg_resources import Distribution as _Dist
  
              compat_dist = cast(_Dist, dist)._dist
              base = AlreadyInstalledCandidate(compat_dist, template, factory=self)
              self._installed_candidate_cache[dist.canonical_name] = base
          if not extras:
              return base
          return self._make_extras_candidate(base, extras)
  
      def _make_candidate_from_link(
          self,
          link: Link,
          extras: FrozenSet[str],
          template: InstallRequirement,
          name: Optional[NormalizedName],
          version: Optional[CandidateVersion],
      ) -> Optional[Candidate]:
          # TODO: Check already installed candidate, and use it if the link and
          # editable flag match.
  
          if link in self._build_failures:
              # We already tried this candidate before, and it does not build.
              # Don't bother trying again.
              return None
  
          if template.editable:
              if link not in self._editable_candidate_cache:
                  try:
                      self._editable_candidate_cache[link] = EditableCandidate(
                          link,
                          template,
                          factory=self,
                          name=name,
                          version=version,
                      )
                  except (InstallationSubprocessError, MetadataInconsistent) as e:
                      logger.warning("Discarding %s. %s", link, e)
                      self._build_failures[link] = e
                      return None
              base: BaseCandidate = self._editable_candidate_cache[link]
          else:
              if link not in self._link_candidate_cache:
                  try:
                      self._link_candidate_cache[link] = LinkCandidate(
                          link,
                          template,
                          factory=self,
                          name=name,
                          version=version,
                      )
                  except (InstallationSubprocessError, MetadataInconsistent) as e:
                      logger.warning("Discarding %s. %s", link, e)
                      self._build_failures[link] = e
                      return None
              base = self._link_candidate_cache[link]
  
          if not extras:
              return base
          return self._make_extras_candidate(base, extras)
  
      def _iter_found_candidates(
          self,
          ireqs: Sequence[InstallRequirement],
          specifier: SpecifierSet,
          hashes: Hashes,
          prefers_installed: bool,
          incompatible_ids: Set[int],
      ) -> Iterable[Candidate]:
          if not ireqs:
              return ()
  
          # The InstallRequirement implementation requires us to give it a
          # "templates". Here we just choose the first requirement to represent
          # all of them.
          # Hopefully the Project model can correct this mismatch in the future.
          template = ireqs[0]
          assert template.req, "Candidates found on index must be PEP 508"
          name = canonicalize_name(template.req.name)
  
          extras: FrozenSet[str] = frozenset()
          for ireq in ireqs:
              assert ireq.req, "Candidates found on index must be PEP 508"
              specifier &= ireq.req.specifier
              hashes &= ireq.hashes(trust_internet=False)
              extras |= frozenset(ireq.extras)
  
          def _get_installed_candidate() -> Optional[Candidate]:
              """Get the candidate for the currently-installed version."""
              # If --force-reinstall is set, we want the version from the index
              # instead, so we "pretend" there is nothing installed.
              if self._force_reinstall:
                  return None
              try:
                  installed_dist = self._installed_dists[name]
              except KeyError:
                  return None
              # Don't use the installed distribution if its version does not fit
              # the current dependency graph.
              if not specifier.contains(installed_dist.version, prereleases=True):
                  return None
              candidate = self._make_candidate_from_dist(
                  dist=installed_dist,
                  extras=extras,
                  template=template,
              )
              # The candidate is a known incompatiblity. Don't use it.
              if id(candidate) in incompatible_ids:
                  return None
              return candidate
  
          def iter_index_candidate_infos() -> Iterator[IndexCandidateInfo]:
              result = self._finder.find_best_candidate(
                  project_name=name,
                  specifier=specifier,
                  hashes=hashes,
              )
              icans = list(result.iter_applicable())
  
              # PEP 592: Yanked releases must be ignored unless only yanked
              # releases can satisfy the version range. So if this is false,
              # all yanked icans need to be skipped.
              all_yanked = all(ican.link.is_yanked for ican in icans)
  
              # PackageFinder returns earlier versions first, so we reverse.
              for ican in reversed(icans):
                  if not all_yanked and ican.link.is_yanked:
                      continue
                  func = functools.partial(
                      self._make_candidate_from_link,
                      link=ican.link,
                      extras=extras,
                      template=template,
                      name=name,
                      version=ican.version,
                  )
                  yield ican.version, func
  
          return FoundCandidates(
              iter_index_candidate_infos,
              _get_installed_candidate(),
              prefers_installed,
              incompatible_ids,
          )
  
      def _iter_explicit_candidates_from_base(
          self,
          base_requirements: Iterable[Requirement],
          extras: FrozenSet[str],
      ) -> Iterator[Candidate]:
          """Produce explicit candidates from the base given an extra-ed package.
  
          :param base_requirements: Requirements known to the resolver. The
              requirements are guaranteed to not have extras.
          :param extras: The extras to inject into the explicit requirements'
              candidates.
          """
          for req in base_requirements:
              lookup_cand, _ = req.get_candidate_lookup()
              if lookup_cand is None:  # Not explicit.
                  continue
              # We've stripped extras from the identifier, and should always
              # get a BaseCandidate here, unless there's a bug elsewhere.
              base_cand = as_base_candidate(lookup_cand)
              assert base_cand is not None, "no extras here"
              yield self._make_extras_candidate(base_cand, extras)
  
      def _iter_candidates_from_constraints(
          self,
          identifier: str,
          constraint: Constraint,
          template: InstallRequirement,
      ) -> Iterator[Candidate]:
          """Produce explicit candidates from constraints.
  
          This creates "fake" InstallRequirement objects that are basically clones
          of what "should" be the templates, but with original_link set to link.
          """
          for link in constraint.links:
              self._fail_if_link_is_unsupported_wheel(link)
              candidate = self._make_candidate_from_link(
                  link,
                  extras=frozenset(),
                  template=install_req_from_link_and_ireq(link, template),
                  name=canonicalize_name(identifier),
                  version=None,
              )
              if candidate:
                  yield candidate
  
      def find_candidates(
          self,
          identifier: str,
          requirements: Mapping[str, Iterator[Requirement]],
          incompatibilities: Mapping[str, Iterator[Candidate]],
          constraint: Constraint,
          prefers_installed: bool,
      ) -> Iterable[Candidate]:
          # Collect basic lookup information from the requirements.
          explicit_candidates: Set[Candidate] = set()
          ireqs: List[InstallRequirement] = []
          for req in requirements[identifier]:
              cand, ireq = req.get_candidate_lookup()
              if cand is not None:
                  explicit_candidates.add(cand)
              if ireq is not None:
                  ireqs.append(ireq)
  
          # If the current identifier contains extras, add explicit candidates
          # from entries from extra-less identifier.
          with contextlib.suppress(InvalidRequirement):
              parsed_requirement = PackagingRequirement(identifier)
              explicit_candidates.update(
                  self._iter_explicit_candidates_from_base(
                      requirements.get(parsed_requirement.name, ()),
                      frozenset(parsed_requirement.extras),
                  ),
              )
  
          # Add explicit candidates from constraints. We only do this if there are
          # kown ireqs, which represent requirements not already explicit. If
          # there are no ireqs, we're constraining already-explicit requirements,
          # which is handled later when we return the explicit candidates.
          if ireqs:
              try:
                  explicit_candidates.update(
                      self._iter_candidates_from_constraints(
                          identifier,
                          constraint,
                          template=ireqs[0],
                      ),
                  )
              except UnsupportedWheel:
                  # If we're constrained to install a wheel incompatible with the
                  # target architecture, no candidates will ever be valid.
                  return ()
  
          # Since we cache all the candidates, incompatibility identification
          # can be made quicker by comparing only the id() values.
          incompat_ids = {id(c) for c in incompatibilities.get(identifier, ())}
  
          # If none of the requirements want an explicit candidate, we can ask
          # the finder for candidates.
          if not explicit_candidates:
              return self._iter_found_candidates(
                  ireqs,
                  constraint.specifier,
                  constraint.hashes,
                  prefers_installed,
                  incompat_ids,
              )
  
          return (
              c
              for c in explicit_candidates
              if id(c) not in incompat_ids
              and constraint.is_satisfied_by(c)
              and all(req.is_satisfied_by(c) for req in requirements[identifier])
          )
  
      def _make_requirement_from_install_req(
          self, ireq: InstallRequirement, requested_extras: Iterable[str]
      ) -> Optional[Requirement]:
          if not ireq.match_markers(requested_extras):
              logger.info(
                  "Ignoring %s: markers '%s' don't match your environment",
                  ireq.name,
                  ireq.markers,
              )
              return None
          if not ireq.link:
              return SpecifierRequirement(ireq)
          self._fail_if_link_is_unsupported_wheel(ireq.link)
          cand = self._make_candidate_from_link(
              ireq.link,
              extras=frozenset(ireq.extras),
              template=ireq,
              name=canonicalize_name(ireq.name) if ireq.name else None,
              version=None,
          )
          if cand is None:
              # There's no way we can satisfy a URL requirement if the underlying
              # candidate fails to build. An unnamed URL must be user-supplied, so
              # we fail eagerly. If the URL is named, an unsatisfiable requirement
              # can make the resolver do the right thing, either backtrack (and
              # maybe find some other requirement that's buildable) or raise a
              # ResolutionImpossible eventually.
              if not ireq.name:
                  raise self._build_failures[ireq.link]
              return UnsatisfiableRequirement(canonicalize_name(ireq.name))
          return self.make_requirement_from_candidate(cand)
  
      def collect_root_requirements(
          self, root_ireqs: List[InstallRequirement]
      ) -> CollectedRootRequirements:
          collected = CollectedRootRequirements([], {}, {})
          for i, ireq in enumerate(root_ireqs):
              if ireq.constraint:
                  # Ensure we only accept valid constraints
                  problem = check_invalid_constraint_type(ireq)
                  if problem:
                      raise InstallationError(problem)
                  if not ireq.match_markers():
                      continue
                  assert ireq.name, "Constraint must be named"
                  name = canonicalize_name(ireq.name)
                  if name in collected.constraints:
                      collected.constraints[name] &= ireq
                  else:
                      collected.constraints[name] = Constraint.from_ireq(ireq)
              else:
                  req = self._make_requirement_from_install_req(
                      ireq,
                      requested_extras=(),
                  )
                  if req is None:
                      continue
                  if ireq.user_supplied and req.name not in collected.user_requested:
                      collected.user_requested[req.name] = i
                  collected.requirements.append(req)
          return collected
  
      def make_requirement_from_candidate(
          self, candidate: Candidate
      ) -> ExplicitRequirement:
          return ExplicitRequirement(candidate)
  
      def make_requirement_from_spec(
          self,
          specifier: str,
          comes_from: InstallRequirement,
          requested_extras: Iterable[str] = (),
      ) -> Optional[Requirement]:
          ireq = self._make_install_req_from_spec(specifier, comes_from)
          return self._make_requirement_from_install_req(ireq, requested_extras)
  
      def make_requires_python_requirement(
          self, specifier: Optional[SpecifierSet]
      ) -> Optional[Requirement]:
          if self._ignore_requires_python or specifier is None:
              return None
          return RequiresPythonRequirement(specifier, self._python_candidate)
  
      def get_wheel_cache_entry(
          self, link: Link, name: Optional[str]
      ) -> Optional[CacheEntry]:
          """Look up the link in the wheel cache.
  
          If ``preparer.require_hashes`` is True, don't use the wheel cache,
          because cached wheels, always built locally, have different hashes
          than the files downloaded from the index server and thus throw false
          hash mismatches. Furthermore, cached wheels at present have
          nondeterministic contents due to file modification times.
          """
          if self._wheel_cache is None or self.preparer.require_hashes:
              return None
          return self._wheel_cache.get_cache_entry(
              link=link,
              package_name=name,
              supported_tags=get_supported(),
          )
  
      def get_dist_to_uninstall(self, candidate: Candidate) -> Optional[BaseDistribution]:
          # TODO: Are there more cases this needs to return True? Editable?
          dist = self._installed_dists.get(candidate.project_name)
          if dist is None:  # Not installed, no uninstallation required.
              return None
  
          # We're installing into global site. The current installation must
          # be uninstalled, no matter it's in global or user site, because the
          # user site installation has precedence over global.
          if not self._use_user_site:
              return dist
  
          # We're installing into user site. Remove the user site installation.
          if dist.in_usersite:
              return dist
  
          # We're installing into user site, but the installed incompatible
          # package is in global site. We can't uninstall that, and would let
          # the new user installation to "shadow" it. But shadowing won't work
          # in virtual environments, so we error out.
          if running_under_virtualenv() and dist.in_site_packages:
              message = (
                  f"Will not install to the user site because it will lack "
                  f"sys.path precedence to {dist.raw_name} in {dist.location}"
              )
              raise InstallationError(message)
          return None
  
      def _report_requires_python_error(
          self, causes: Sequence["ConflictCause"]
      ) -> UnsupportedPythonVersion:
          assert causes, "Requires-Python error reported with no cause"
  
          version = self._python_candidate.version
  
          if len(causes) == 1:
              specifier = str(causes[0].requirement.specifier)
              message = (
                  f"Package {causes[0].parent.name!r} requires a different "
                  f"Python: {version} not in {specifier!r}"
              )
              return UnsupportedPythonVersion(message)
  
          message = f"Packages require a different Python. {version} not in:"
          for cause in causes:
              package = cause.parent.format_for_error()
              specifier = str(cause.requirement.specifier)
              message += f"\n{specifier!r} (required by {package})"
          return UnsupportedPythonVersion(message)
  
      def _report_single_requirement_conflict(
          self, req: Requirement, parent: Optional[Candidate]
      ) -> DistributionNotFound:
          if parent is None:
              req_disp = str(req)
          else:
              req_disp = f"{req} (from {parent.name})"
  
          cands = self._finder.find_all_candidates(req.project_name)
          versions = [str(v) for v in sorted({c.version for c in cands})]
  
          logger.critical(
              "Could not find a version that satisfies the requirement %s "
              "(from versions: %s)",
              req_disp,
              ", ".join(versions) or "none",
          )
          if str(req) == "requirements.txt":
              logger.info(
                  "HINT: You are attempting to install a package literally "
                  'named "requirements.txt" (which cannot exist). Consider '
                  "using the '-r' flag to install the packages listed in "
                  "requirements.txt"
              )
  
          return DistributionNotFound(f"No matching distribution found for {req}")
  
      def get_installation_error(
          self,
          e: "ResolutionImpossible[Requirement, Candidate]",
          constraints: Dict[str, Constraint],
      ) -> InstallationError:
  
          assert e.causes, "Installation error reported with no cause"
  
          # If one of the things we can't solve is "we need Python X.Y",
          # that is what we report.
          requires_python_causes = [
              cause
              for cause in e.causes
              if isinstance(cause.requirement, RequiresPythonRequirement)
              and not cause.requirement.is_satisfied_by(self._python_candidate)
          ]
          if requires_python_causes:
              # The comprehension above makes sure all Requirement instances are
              # RequiresPythonRequirement, so let's cast for convinience.
              return self._report_requires_python_error(
                  cast("Sequence[ConflictCause]", requires_python_causes),
              )
  
          # Otherwise, we have a set of causes which can't all be satisfied
          # at once.
  
          # The simplest case is when we have *one* cause that can't be
          # satisfied. We just report that case.
          if len(e.causes) == 1:
              req, parent = e.causes[0]
              if req.name not in constraints:
                  return self._report_single_requirement_conflict(req, parent)
  
          # OK, we now have a list of requirements that can't all be
          # satisfied at once.
  
          # A couple of formatting helpers
          def text_join(parts: List[str]) -> str:
              if len(parts) == 1:
                  return parts[0]
  
              return ", ".join(parts[:-1]) + " and " + parts[-1]
  
          def describe_trigger(parent: Candidate) -> str:
              ireq = parent.get_install_requirement()
              if not ireq or not ireq.comes_from:
                  return f"{parent.name}=={parent.version}"
              if isinstance(ireq.comes_from, InstallRequirement):
                  return str(ireq.comes_from.name)
              return str(ireq.comes_from)
  
          triggers = set()
          for req, parent in e.causes:
              if parent is None:
                  # This is a root requirement, so we can report it directly
                  trigger = req.format_for_error()
              else:
                  trigger = describe_trigger(parent)
              triggers.add(trigger)
  
          if triggers:
              info = text_join(sorted(triggers))
          else:
              info = "the requested packages"
  
          msg = (
              "Cannot install {} because these package versions "
              "have conflicting dependencies.".format(info)
          )
          logger.critical(msg)
          msg = "\nThe conflict is caused by:"
  
          relevant_constraints = set()
          for req, parent in e.causes:
              if req.name in constraints:
                  relevant_constraints.add(req.name)
              msg = msg + "\n    "
              if parent:
                  msg = msg + f"{parent.name} {parent.version} depends on "
              else:
                  msg = msg + "The user requested "
              msg = msg + req.format_for_error()
          for key in relevant_constraints:
              spec = constraints[key].specifier
              msg += f"\n    The user requested (constraint) {key}{spec}"
  
          msg = (
              msg
              + "\n\n"
              + "To fix this you could try to:\n"
              + "1. loosen the range of package versions you've specified\n"
              + "2. remove package versions to allow pip attempt to solve "
              + "the dependency conflict\n"
          )
  
          logger.info(msg)
  
          return DistributionNotFound(
              "ResolutionImpossible: for help visit "
              "https://pip.pypa.io/en/latest/user_guide/"
              "#fixing-conflicting-dependencies"
          )
  