explain why version cannot be retrieved when Requires-Python is not satisfied

news/9615.feature.rst

@@ -0,0 +1 @@
+Explains why specified version cannot be retrieved when *Requires-Python* is not satisfied.

src/pip/_internal/index/package_finder.py

@@ -222,9 +222,8 @@ def evaluate_link(self, link: Link) -> Tuple[bool, Optional[str]]:
             ignore_requires_python=self._ignore_requires_python,
         )
         if not supports_python:
-            # Return None for the reason text to suppress calling
-            # _log_skipped_link().
-            return (False, None)
+            reason = f"{version} Requires-Python {link.requires_python}"
+            return (False, reason)
 
         logger.debug("Found link %s, version: %s", link, version)
 
@@ -610,7 +609,7 @@ def __init__(
         self.format_control = format_control
 
         # These are boring links that have already been logged somehow.
-        self._logged_links: Set[Link] = set()
+        self._logged_links: Set[Tuple[Link, str]] = set()
 
     # Don't include an allow_yanked default value to make sure each call
     # site considers whether yanked releases are allowed. This also causes
@@ -690,6 +689,11 @@ def prefer_binary(self) -> bool:
     def set_prefer_binary(self) -> None:
         self._candidate_prefs.prefer_binary = True
 
+    def skipped_links_requires_python(self) -> List[str]:
+        return sorted(
+            {reason for _, reason in self._logged_links if "Requires-Python" in reason}
+        )
+
     def make_link_evaluator(self, project_name: str) -> LinkEvaluator:
         canonical_name = canonicalize_name(project_name)
         formats = self.format_control.get_allowed_formats(canonical_name)
@@ -720,11 +724,11 @@ def _sort_links(self, links: Iterable[Link]) -> List[Link]:
         return no_eggs + eggs
 
     def _log_skipped_link(self, link: Link, reason: str) -> None:
-        if link not in self._logged_links:
+        if (link, reason) not in self._logged_links:
             # Put the link at the end so the reason is more visible and because
             # the link string is usually very long.
             logger.debug("Skipping link: %s: %s", reason, link)
-            self._logged_links.add(link)
+            self._logged_links.add((link, reason))
 
     def get_install_candidate(
         self, link_evaluator: LinkEvaluator, link: Link
@@ -735,8 +739,7 @@ def get_install_candidate(
         """
         is_candidate, result = link_evaluator.evaluate_link(link)
         if not is_candidate:
-            if result:
-                self._log_skipped_link(link, reason=result)
+            self._log_skipped_link(link, result)
             return None
 
         return InstallationCandidate(

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

@@ -617,8 +617,15 @@ def _report_single_requirement_conflict(
             req_disp = f"{req} (from {parent.name})"
 
         cands = self._finder.find_all_candidates(req.project_name)
+        skips = self._finder.skipped_links_requires_python()
         versions = [str(v) for v in sorted({c.version for c in cands})]
 
+        if skips:
+            logger.critical(
+                "Ignored the following versions that require a different python "
+                "version: %s",
+                "; ".join(skips) or "none",
+            )
         logger.critical(
             "Could not find a version that satisfies the requirement %s "
             "(from versions: %s)",

tests/unit/test_index.py

@@ -120,7 +120,7 @@ class TestLinkEvaluator:
         [
             ((3, 6, 5), False, (True, "1.12")),
             # Test an incompatible Python.
-            ((3, 6, 4), False, (False, None)),
+            ((3, 6, 4), False, (False, "1.12 Requires-Python == 3.6.5")),
             # Test an incompatible Python with ignore_requires_python=True.
             ((3, 6, 4), True, (True, "1.12")),
         ],