WIP: add --prefer-minimum-versions flag

news/8085.feature

@@ -0,0 +1,5 @@
+Add a ``--prefer-minimum-versions`` command line flag to tell pip to
+use older versions instead of newer versions for all
+dependencies. This is useful for running test suites using the "lower
+bounds" of requirements to ensure that they are accurate. The flag is
+only available when the new resolver is enabled.

src/pip/_internal/cli/cmdoptions.py

@@ -930,6 +930,17 @@ def check_list_path_option(options):
 )  # type: Callable[..., Option]
 
 
+prefer_minimum_versions = partial(
+    Option,
+    '--prefer-minimum-versions',
+    dest='prefer_minimum_versions',
+    action='store_true',
+    default=False,
+    help=SUPPRESS_HELP,  # TODO: Enable this when the resolver actually works.
+    # help='Use the lowest version that matches a requirement.',
+)  # type: Callable[..., Option]
+
+
 ##########
 # groups #
 ##########

src/pip/_internal/cli/req_command.py

@@ -200,6 +200,7 @@ def __init__(self, *args, **kw):
         super(RequirementCommand, self).__init__(*args, **kw)
 
         self.cmd_opts.add_option(cmdoptions.no_clean())
+        self.cmd_opts.add_option(cmdoptions.prefer_minimum_versions())
 
     @staticmethod
     def make_requirement_preparer(
@@ -274,6 +275,7 @@ def make_resolver(
                 force_reinstall=force_reinstall,
                 upgrade_strategy=upgrade_strategy,
                 py_version_info=py_version_info,
+                prefer_minimum_versions=options.prefer_minimum_versions,
             )
         import pip._internal.resolution.legacy.resolver
         return pip._internal.resolution.legacy.resolver.Resolver(

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

@@ -16,10 +16,12 @@ def __init__(
         self,
         factory,  # type: Factory
         ignore_dependencies,  # type: bool
+        prefer_minimum_versions=False,  # type: bool
     ):
         # type: (...) -> None
         self._factory = factory
         self._ignore_dependencies = ignore_dependencies
+        self._prefer_minimum_versions = prefer_minimum_versions
 
     def get_install_requirement(self, c):
         # type: (Candidate) -> Optional[InstallRequirement]
@@ -36,12 +38,14 @@ def get_preference(
         information  # type: Sequence[Tuple[Requirement, Candidate]]
     ):
         # type: (...) -> Any
-        # Use the "usual" value for now
         return len(candidates)
 
     def find_matches(self, requirement):
         # type: (Requirement) -> Sequence[Candidate]
-        return requirement.find_matches()
+        results = requirement.find_matches()
+        if self._prefer_minimum_versions:
+            results = list(reversed(results))
+        return results
 
     def is_satisfied_by(self, requirement, candidate):
         # type: (Requirement, Candidate) -> bool

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

@@ -43,6 +43,7 @@ def __init__(
         force_reinstall,  # type: bool
         upgrade_strategy,  # type: str
         py_version_info=None,  # type: Optional[Tuple[int, ...]]
+        prefer_minimum_versions=False,  # type: bool
     ):
         super(Resolver, self).__init__()
         self.factory = Factory(
@@ -55,6 +56,7 @@ def __init__(
             py_version_info=py_version_info,
         )
         self.ignore_dependencies = ignore_dependencies
+        self.prefer_minimum_versions = prefer_minimum_versions
         self._result = None  # type: Optional[Result]
 
     def resolve(self, root_reqs, check_supported_wheels):
@@ -67,6 +69,7 @@ def resolve(self, root_reqs, check_supported_wheels):
         provider = PipProvider(
             factory=self.factory,
             ignore_dependencies=self.ignore_dependencies,
+            prefer_minimum_versions=self.prefer_minimum_versions,
         )
         reporter = BaseReporter()
         resolver = RLResolver(provider, reporter)