Make new resolver process Requires-Python before other dependencies

news/11142.bugfix.rst

@@ -0,0 +1,2 @@
+New resolver: Fix a regression that prevents the ``Requires-Python`` metadata
+to be correctly checked before Python package dependencies.

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

@@ -1,3 +1,4 @@
+import functools
 import logging
 import sys
 from typing import TYPE_CHECKING, Any, FrozenSet, Iterable, Optional, Tuple, Union, cast
@@ -141,7 +142,6 @@ class exposes appropriate information to the resolver.
         found remote link (e.g. from pypi.org).
     """
 
-    dist: BaseDistribution
     is_installed = False
 
     def __init__(
@@ -159,7 +159,6 @@ def __init__(
         self._ireq = ireq
         self._name = name
         self._version = version
-        self.dist = self._prepare()
 
     def __str__(self) -> str:
         return f"{self.name} {self.version}"
@@ -226,7 +225,10 @@ def _check_metadata_consistency(self, dist: BaseDistribution) -> None:
                 str(dist.version),
             )
 
-    def _prepare(self) -> BaseDistribution:
+    @functools.lru_cache(maxsize=None)
+    def __prepare_dist_property(self) -> BaseDistribution:
+        # TODO: When we drop python 3.7 support, move this to 'dist' and use
+        # functools.cached_property instead of lru_cache.
         try:
             dist = self._prepare_distribution()
         except HashError as e:
@@ -243,11 +245,15 @@ def _prepare(self) -> BaseDistribution:
         self._check_metadata_consistency(dist)
         return dist
 
+    @property
+    def dist(self) -> BaseDistribution:
+        return self.__prepare_dist_property()
+
     def iter_dependencies(self, with_requires: bool) -> Iterable[Optional[Requirement]]:
+        yield self._factory.make_requires_python_requirement(self.dist.requires_python)
         requires = self.dist.iter_dependencies() if with_requires else ()
         for r in requires:
             yield self._factory.make_requirement_from_spec(str(r), self._ireq)
-        yield self._factory.make_requires_python_requirement(self.dist.requires_python)
 
     def get_install_requirement(self) -> Optional[InstallRequirement]:
         return self._ireq

tests/functional/test_new_resolver_errors.py

@@ -101,32 +101,32 @@ def test_new_resolver_checks_requires_python_before_dependencies(
 ) -> None:
     incompatible_python = "<{0.major}.{0.minor}".format(sys.version_info)
 
-    pkg_dep = create_basic_wheel_for_package(
+    pkgdep = create_basic_wheel_for_package(
         script,
-        name="pkg-dep",
+        name="pkgdep",
         version="1",
     )
     create_basic_wheel_for_package(
         script,
-        name="pkg-root",
+        name="pkgroot",
         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()}"],
+        depends=[
+            f"pkgdep@{pathlib.Path(pkgdep).as_uri()}",
+        ],
         requires_python=incompatible_python,
     )
 
-    result = script.pip(
+    r = script.pip(
         "install",
         "--no-cache-dir",
         "--no-index",
         "--find-links",
         script.scratch_path,
-        "pkg-root",
+        "pkgroot",
         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)
+    # Resolution should fail because of pkgroot's Requires-Python.
+    # This is done before dependencies so pkgdep should never be pulled.
+    assert incompatible_python in r.stderr, str(r)
+    assert "pkgdep" not in r.stderr and "pkgdep" not in r.stdout, str(r)