Improve the debug log message when installing an incompatible wheel.

news/6121.feature

@@ -0,0 +1,2 @@
+Include the wheel's tags in the log message explanation when a candidate
+wheel link is found incompatible.

src/pip/_internal/index.py

@@ -384,7 +384,15 @@ class CandidateEvaluator(object):
                     return (False, reason)
 
                 if not self._is_wheel_supported(wheel):
-                    return (False, 'it is not compatible with this Python')
+                    # Include the wheel's tags in the reason string to
+                    # simplify troubleshooting compatibility issues.
+                    file_tags = wheel.get_formatted_file_tags()
+                    reason = (
+                        "none of the wheel's tags match: {}".format(
+                            ', '.join(file_tags)
+                        )
+                    )
+                    return (False, reason)
 
                 version = wheel.version
 
@@ -1066,7 +1074,9 @@ class PackageFinder(object):
     def _log_skipped_link(self, link, reason):
         # type: (Link, str) -> None
         if link not in self._logged_links:
-            logger.debug('Skipping link %s; %s', link, reason)
+            # 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)
 
     def get_install_candidate(self, link, search):

src/pip/_internal/wheel.py

@@ -663,6 +663,16 @@ def check_compatibility(version, name):
         )
 
 
+def format_tag(file_tag):
+    # type: (Tuple[str, ...]) -> str
+    """
+    Format three tags in the form "<python_tag>-<abi_tag>-<platform_tag>".
+
+    :param file_tag: A 3-tuple of tags (python_tag, abi_tag, platform_tag).
+    """
+    return '-'.join(file_tag)
+
+
 class Wheel(object):
     """A wheel file"""
 
@@ -702,6 +712,13 @@ class Wheel(object):
             for y in self.abis for z in self.plats
         }
 
+    def get_formatted_file_tags(self):
+        # type: () -> List[str]
+        """
+        Return the wheel's tags as a sorted list of strings.
+        """
+        return sorted(format_tag(tag) for tag in self.file_tags)
+
     def support_index_min(self, tags=None):
         # type: (Optional[List[Pep425Tag]]) -> Optional[int]
         """

tests/functional/test_install_config.py

@@ -105,7 +105,10 @@ def test_command_line_append_flags(script, virtualenv, data):
         "Analyzing links from page https://test.pypi.org"
         in result.stdout
     )
-    assert "Skipping link %s" % data.find_links in result.stdout
+    assert (
+        'Skipping link: not a file: {}'.format(data.find_links) in
+        result.stdout
+    ), 'stdout: {}'.format(result.stdout)
 
 
 @pytest.mark.network
@@ -127,7 +130,10 @@ def test_command_line_appends_correctly(script, data):
         "Analyzing links from page https://test.pypi.org"
         in result.stdout
     ), result.stdout
-    assert "Skipping link %s" % data.find_links in result.stdout
+    assert (
+        'Skipping link: not a file: {}'.format(data.find_links) in
+        result.stdout
+    ), 'stdout: {}'.format(result.stdout)
 
 
 def test_config_file_override_stack(script, virtualenv):

tests/unit/test_finder.py

@@ -133,10 +133,7 @@ class TestWheel:
         with pytest.raises(DistributionNotFound):
             finder.find_requirement(req, True)
 
-        assert (
-            "invalid.whl; invalid wheel filename"
-            in caplog.text
-        )
+        assert 'Skipping link: invalid wheel filename:' in caplog.text
 
     def test_not_find_wheel_not_supported(self, data, monkeypatch):
         """

tests/unit/test_index.py

@@ -130,6 +130,24 @@ class TestCandidateEvaluator:
         actual = evaluator.evaluate_link(link, search=search)
         assert actual == expected
 
+    def test_evaluate_link__incompatible_wheel(self):
+        """
+        Test an incompatible wheel.
+        """
+        link = Link('https://example.com/sample-1.0-py2.py3-none-any.whl')
+        search = Search(
+            supplied='sample', canonical='sample', formats=['binary'],
+        )
+        # Pass an empty list for the valid tags to make sure nothing matches.
+        evaluator = CandidateEvaluator(
+            [], py_version_info=(3, 6, 4),
+        )
+        actual = evaluator.evaluate_link(link, search=search)
+        expected = (
+            False, "none of the wheel's tags match: py2-none-any, py3-none-any"
+        )
+        assert actual == expected
+
 
 def test_sort_locations_file_expand_dir(data):
     """

tests/unit/test_wheel.py

@@ -68,6 +68,15 @@ def make_test_install_req(base_name=None):
     return req
 
 
+@pytest.mark.parametrize('file_tag, expected', [
+    (('py27', 'none', 'any'), 'py27-none-any'),
+    (('cp33', 'cp32dmu', 'linux_x86_64'), 'cp33-cp32dmu-linux_x86_64'),
+])
+def test_format_tag(file_tag, expected):
+    actual = wheel.format_tag(file_tag)
+    assert actual == expected
+
+
 @pytest.mark.parametrize(
     "base_name, autobuilding, cache_available, expected",
     [