Check Requires-Python before other dependencies

This makes the resolver fail quicker when there's a interpreter
version conflict, and avoid additional (useless) downloads.

Author: uranusjr

news/9925.feature.rst

@@ -0,0 +1,3 @@
+New resolver: A distribution's ``Requires-Python`` metadata is now checked
+before its Python dependencies. This makes the resolver fail quicker when
+there's an interpreter version conflict.

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

@@ -32,6 +32,9 @@
     "LinkCandidate",
 ]
 
+# Avoid conflicting with the PyPI package "Python".
+REQUIRES_PYTHON_IDENTIFIER = cast(NormalizedName, "<Python from Requires-Python>")
+
 
 def as_base_candidate(candidate: Candidate) -> Optional[BaseCandidate]:
     """The runtime version of BaseCandidate."""
@@ -578,13 +581,12 @@ def __str__(self):
     @property
     def project_name(self):
         # type: () -> NormalizedName
-        # Avoid conflicting with the PyPI package "Python".
-        return cast(NormalizedName, "<Python from Requires-Python>")
+        return REQUIRES_PYTHON_IDENTIFIER
 
     @property
     def name(self):
         # type: () -> str
-        return self.project_name
+        return REQUIRES_PYTHON_IDENTIFIER
 
     @property
     def version(self):

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

@@ -3,6 +3,7 @@
 from pip._vendor.resolvelib.providers import AbstractProvider
 
 from .base import Candidate, Constraint, Requirement
+from .candidates import REQUIRES_PYTHON_IDENTIFIER
 from .factory import Factory
 
 if TYPE_CHECKING:
@@ -121,6 +122,10 @@ def _get_restrictive_rating(requirements):
         rating = _get_restrictive_rating(r for r, _ in information[identifier])
         order = self._user_requested.get(identifier, float("inf"))
 
+        # Requires-Python has only one candidate and the check is basically
+        # free, so we always do it first to avoid needless work if it fails.
+        requires_python = identifier == REQUIRES_PYTHON_IDENTIFIER
+
         # HACK: Setuptools have a very long and solid backward compatibility
         # track record, and extremely few projects would request a narrow,
         # non-recent version range of it since that would break a lot things.
@@ -131,7 +136,7 @@ def _get_restrictive_rating(requirements):
         # while we work on "proper" branch pruning techniques.
         delay_this = identifier == "setuptools"
 
-        return (delay_this, rating, order, identifier)
+        return (not requires_python, delay_this, rating, order, identifier)
 
     def find_matches(
         self,

tests/functional/test_new_resolver_errors.py

@@ -1,3 +1,4 @@
+import pathlib
 import sys
 
 from tests.lib import create_basic_wheel_for_package, create_test_package_with_setup
@@ -73,3 +74,34 @@ def test_new_resolver_requires_python_error(script):
     # conflict, not the compatible one.
     assert incompatible_python in result.stderr, str(result)
     assert compatible_python not in result.stderr, str(result)
+
+
+def test_new_resolver_checks_requires_python_before_dependencies(script):
+    incompatible_python = "<{0.major}.{0.minor}".format(sys.version_info)
+
+    pkg_dep = create_basic_wheel_for_package(
+        script,
+        name="pkg-dep",
+        version="1",
+    )
+    create_basic_wheel_for_package(
+        script,
+        name="pkg-root",
+        version="1",
+        # Refer the dependency by URL to prioritise it as much as possible,
+        # to test that Requires-Python is *still* inspected first.
+        depends=[f"pkg-dep@{pathlib.Path(pkg_dep).as_uri()}"],
+        requires_python=incompatible_python,
+    )
+
+    result = script.pip(
+        "install", "--no-cache-dir",
+        "--no-index", "--find-links", script.scratch_path,
+        "pkg-root",
+        expect_error=True,
+    )
+
+    # Resolution should fail because of pkg-a's Requires-Python.
+    # This check should be done before pkg-b, so pkg-b should never be pulled.
+    assert incompatible_python in result.stderr, str(result)
+    assert "pkg-b" not in result.stderr, str(result)