🐛 fix(discovery): match prerelease versions against major.minor specs

Python prerelease versions like 3.15.0a6 were incorrectly failing to match
version specifiers like >=3.15. This broke testing of prereleases in Fedora
and other environments that build libraries against alpha/beta Python versions.

The root cause was comparing the full version string "3.15.0a6" against
normalized specifier versions without accounting for precision. For a spec
like >=3.15 (two components), comparing against "3.15.0a6" failed because
PEP 440 defines prereleases as less than final releases.

The fix determines precision from the specifier's version string by counting
dots, then only includes the prerelease suffix when either the precision is
3 (full version like >=3.15.0) or the specifier itself contains a prerelease
marker (like >=3.15.0a1). This allows >=3.15 to match 3.15.0a6 by comparing
"3.15" to "3.15", while >=3.15.0 correctly rejects it by comparing "3.15.0a6"
to "3.15.0".

Fixes #45

Author: gaborbernat

src/python_discovery/_py_info.py

@@ -469,12 +469,23 @@ def _satisfies_version_specifier(self, spec: PythonSpec) -> bool:
         if spec.version_specifier is None:  # pragma: no cover
             return True
         version_info = self.version_info
-        release = f"{version_info.major}.{version_info.minor}.{version_info.micro}"
-        if version_info.releaselevel != "final":
-            suffix = {"alpha": "a", "beta": "b", "candidate": "rc"}.get(version_info.releaselevel)
-            if suffix is not None:  # pragma: no branch # releaselevel is always alpha/beta/candidate here
+        for specifier in spec.version_specifier:
+            numeric_version = specifier.version_str
+            for prefix in ("rc", "b", "a"):
+                if prefix in numeric_version:
+                    numeric_version = numeric_version.split(prefix)[0]
+                    break
+            precision = numeric_version.count(".") + 1
+            release = ".".join(str(c) for c in [version_info.major, version_info.minor, version_info.micro][:precision])
+            if (
+                version_info.releaselevel != "final"
+                and (precision == 3 or specifier.version.pre_type is not None)  # noqa: PLR2004
+                and (suffix := {"alpha": "a", "beta": "b", "candidate": "rc"}.get(version_info.releaselevel))
+            ):
                 release = f"{release}{suffix}{version_info.serial}"
-        return spec.version_specifier.contains(release)
+            if not specifier.contains(release):
+                return False
+        return True
 
     _current_system = None
     _current = None

tests/test_py_info_extra.py

@@ -276,25 +276,29 @@ def test_satisfies_version_specifier_fails() -> None:
     assert CURRENT.satisfies(spec, impl_must_match=False) is False
 
 
-def test_satisfies_prerelease_version() -> None:
+@pytest.mark.parametrize(
+    ("version_info", "spec_str", "expected"),
+    [
+        pytest.param(VersionInfo(3, 14, 0, "alpha", 1), ">=3.14.0a1", True, id="alpha_match_exact"),
+        pytest.param(VersionInfo(3, 14, 0, "beta", 1), ">=3.14.0b1", True, id="beta_match_exact"),
+        pytest.param(VersionInfo(3, 14, 0, "candidate", 1), ">=3.14.0rc1", True, id="rc_match_exact"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 6), ">=3.15", True, id="prerelease_match_major_minor"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 6), ">=3.15.0", False, id="prerelease_not_match_full_precision"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 5), "<3.15.0a6", True, id="earlier_prerelease_less_than"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 6), "<3.15.0a6", False, id="prerelease_not_less_than_itself"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 6), ">=3.15.0a6", True, id="prerelease_match_itself"),
+        pytest.param(VersionInfo(3, 15, 0, "alpha", 6), ">=3.15.0a7", False, id="prerelease_not_match_later"),
+        pytest.param(VersionInfo(3, 15, 0, "final", 0), ">=3.15.0a6", True, id="final_greater_than_prerelease"),
+        pytest.param(VersionInfo(3, 15, 0, "final", 0), "<3.15.0a6", False, id="final_not_less_than_prerelease"),
+        pytest.param(VersionInfo(3, 15, 0, "final", 0), ">=3.15", True, id="final_match_major_minor"),
+        pytest.param(VersionInfo(3, 15, 1, "alpha", 1), ">=3.15.0", True, id="later_micro_prerelease_match"),
+    ],
+)
+def test_satisfies_version_specifier_prerelease(version_info: VersionInfo, spec_str: str, expected: bool) -> None:
     info = copy.deepcopy(CURRENT)
-    info.version_info = VersionInfo(3, 14, 0, "alpha", 1)
-    spec = PythonSpec.from_string_spec(">=3.14.0a1")
-    assert info.satisfies(spec, impl_must_match=False) is True
-
-
-def test_satisfies_prerelease_beta() -> None:
-    info = copy.deepcopy(CURRENT)
-    info.version_info = VersionInfo(3, 14, 0, "beta", 1)
-    spec = PythonSpec.from_string_spec(">=3.14.0b1")
-    assert info.satisfies(spec, impl_must_match=False) is True
-
-
-def test_satisfies_prerelease_candidate() -> None:
-    info = copy.deepcopy(CURRENT)
-    info.version_info = VersionInfo(3, 14, 0, "candidate", 1)
-    spec = PythonSpec.from_string_spec(">=3.14.0rc1")
-    assert info.satisfies(spec, impl_must_match=False) is True
+    info.version_info = version_info
+    spec = PythonSpec.from_string_spec(spec_str)
+    assert info.satisfies(spec, impl_must_match=False) is expected
 
 
 def test_satisfies_path_not_abs_basename_match() -> None: