Improve install conflict warning

src/pip/_internal/commands/install.py

@@ -21,6 +21,7 @@
 from pip._internal.operations.check import check_install_conflicts
 from pip._internal.req import install_given_reqs
 from pip._internal.req.req_tracker import get_requirement_tracker
+from pip._internal.utils.datetime import today_is_later_than
 from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.distutils_args import parse_distutils_args
 from pip._internal.utils.filesystem import test_writable_dir
@@ -444,7 +445,10 @@ def run(self, options, args):
                 items.append(item)
 
             if conflicts is not None:
-                self._warn_about_conflicts(conflicts)
+                self._warn_about_conflicts(
+                    conflicts,
+                    new_resolver='2020-resolver' in options.features_enabled,
+                )
 
             installed_desc = ' '.join(items)
             if installed_desc:
@@ -536,27 +540,68 @@ def _determine_conflicts(self, to_install):
             )
             return None
 
-    def _warn_about_conflicts(self, conflict_details):
-        # type: (ConflictDetails) -> None
+    def _warn_about_conflicts(self, conflict_details, new_resolver):
+        # type: (ConflictDetails, bool) -> None
         package_set, (missing, conflicting) = conflict_details
+        if not missing and not conflicting:
+            return
+
+        parts = []  # type: List[str]
+        if not new_resolver:
+            parts.append(
+                "After October 2020 you may experience errors when installing "
+                "or updating packages. This is because pip will change the "
+                "way that it resolves dependency conflicts.\n"
+            )
+            parts.append(
+                "We recommend you use --use-feature=2020-resolver to test "
+                "your packages with the new resolver before it becomes the "
+                "default.\n"
+            )
+        elif not today_is_later_than(year=2020, month=7, day=31):
+            # NOTE: trailing newlines here are intentional
+            parts.append(
+                "Pip will install or upgrade your package(s) and its "
+                "dependencies without taking into account other packages you "
+                "already have installed. This may cause an uncaught "
+                "dependency conflict.\n"
+            )
+            form_link = "https://forms.gle/cWKMoDs8sUVE29hz9"
+            parts.append(
+                "If you would like pip to take your other packages into "
+                "account, please tell us here: {}\n".format(form_link)
+            )
 
         # NOTE: There is some duplication here, with commands/check.py
         for project_name in missing:
             version = package_set[project_name][0]
             for dependency in missing[project_name]:
-                logger.critical(
-                    "%s %s requires %s, which is not installed.",
-                    project_name, version, dependency[1],
+                message = (
+                    "{name} {version} requires {requirement}, "
+                    "which is not installed."
+                ).format(
+                    name=project_name,
+                    version=version,
+                    requirement=dependency[1],
                 )
+                parts.append(message)
 
         for project_name in conflicting:
             version = package_set[project_name][0]
             for dep_name, dep_version, req in conflicting[project_name]:
-                logger.critical(
-                    "%s %s has requirement %s, but you'll have %s %s which is "
-                    "incompatible.",
-                    project_name, version, req, dep_name, dep_version,
+                message = (
+                    "{name} {version} requires {requirement}, but you'll have "
+                    "{dep_name} {dep_version} which is incompatible."
+                ).format(
+                    name=project_name,
+                    version=version,
+                    requirement=req,
+                    dep_name=dep_name,
+                    dep_version=dep_version,
                 )
+                parts.append(message)
+
+        logger.critical("\n".join(parts))
 
 
 def get_lib_location_guesses(

src/pip/_internal/utils/datetime.py

@@ -0,0 +1,14 @@
+"""For when pip wants to check the date or time.
+"""
+
+from __future__ import absolute_import
+
+import datetime
+
+
+def today_is_later_than(year, month, day):
+    # type: (int, int, int) -> bool
+    today = datetime.date.today()
+    given = datetime.date(year, month, day)
+
+    return today > given

tests/functional/test_check.py

@@ -3,9 +3,11 @@
 
 def matches_expected_lines(string, expected_lines):
     # Ignore empty lines
-    output_lines = set(filter(None, string.splitlines()))
-    # Match regardless of order
-    return set(output_lines) == set(expected_lines)
+    output_lines = list(filter(None, string.splitlines()))
+    # We'll match the last n lines, given n lines to match.
+    last_few_output_lines = output_lines[-len(expected_lines):]
+    # And order does not matter
+    return set(last_few_output_lines) == set(expected_lines)
 
 
 def test_basic_check_clean(script):

tests/functional/test_install.py

@@ -1697,7 +1697,7 @@ def test_install_conflict_results_in_warning(script, data):
     result2 = script.pip(
         'install', '--no-index', pkgB_path, allow_stderr_error=True,
     )
-    assert "pkga 1.0 has requirement pkgb==1.0" in result2.stderr, str(result2)
+    assert "pkga 1.0 requires pkgb==1.0" in result2.stderr, str(result2)
     assert "Successfully installed pkgB-2.0" in result2.stdout, str(result2)
 
 

tests/functional/test_install_check.py

@@ -1,14 +1,11 @@
 from tests.lib import create_test_package_with_setup
 
 
-def matches_expected_lines(string, expected_lines, exact=True):
-    if exact:
-        return set(string.splitlines()) == set(expected_lines)
-    # If not exact, check that all expected lines are present
+def contains_expected_lines(string, expected_lines):
     return set(expected_lines) <= set(string.splitlines())
 
 
-def test_check_install_canonicalization(script, deprecated_python):
+def test_check_install_canonicalization(script):
     pkga_path = create_test_package_with_setup(
         script,
         name='pkgA',
@@ -38,11 +35,10 @@ def test_check_install_canonicalization(script, deprecated_python):
         allow_stderr_error=True,
     )
     expected_lines = [
-        "ERROR: pkga 1.0 requires SPECIAL.missing, which is not installed.",
+        "pkga 1.0 requires SPECIAL.missing, which is not installed.",
     ]
     # Deprecated python versions produce an extra warning on stderr
-    assert matches_expected_lines(
-        result.stderr, expected_lines, exact=not deprecated_python)
+    assert contains_expected_lines(result.stderr, expected_lines)
     assert result.returncode == 0
 
     # Install the second missing package and expect that there is no warning
@@ -51,21 +47,19 @@ def test_check_install_canonicalization(script, deprecated_python):
     result = script.pip(
         'install', '--no-index', special_path, '--quiet',
     )
-    assert matches_expected_lines(
-        result.stderr, [], exact=not deprecated_python)
+    assert "requires" not in result.stderr
     assert result.returncode == 0
 
     # Double check that all errors are resolved in the end
     result = script.pip('check')
     expected_lines = [
         "No broken requirements found.",
     ]
-    assert matches_expected_lines(result.stdout, expected_lines)
+    assert contains_expected_lines(result.stdout, expected_lines)
     assert result.returncode == 0
 
 
-def test_check_install_does_not_warn_for_out_of_graph_issues(
-        script, deprecated_python):
+def test_check_install_does_not_warn_for_out_of_graph_issues(script):
     pkg_broken_path = create_test_package_with_setup(
         script,
         name='broken',
@@ -85,33 +79,30 @@ def test_check_install_does_not_warn_for_out_of_graph_issues(
 
     # Install a package without it's dependencies
     result = script.pip('install', '--no-index', pkg_broken_path, '--no-deps')
-    # Deprecated python versions produce an extra warning on stderr
-    assert matches_expected_lines(
-        result.stderr, [], exact=not deprecated_python)
+    assert "requires" not in result.stderr
 
     # Install conflict package
     result = script.pip(
         'install', '--no-index', pkg_conflict_path, allow_stderr_error=True,
     )
-    assert matches_expected_lines(result.stderr, [
-        "ERROR: broken 1.0 requires missing, which is not installed.",
+    assert contains_expected_lines(result.stderr, [
+        "broken 1.0 requires missing, which is not installed.",
         (
-            "ERROR: broken 1.0 has requirement conflict<1.0, but "
+            "broken 1.0 requires conflict<1.0, but "
             "you'll have conflict 1.0 which is incompatible."
         ),
-    ], exact=not deprecated_python)
+    ])
 
     # Install unrelated package
     result = script.pip(
         'install', '--no-index', pkg_unrelated_path, '--quiet',
     )
     # should not warn about broken's deps when installing unrelated package
-    assert matches_expected_lines(
-        result.stderr, [], exact=not deprecated_python)
+    assert "requires" not in result.stderr
 
     result = script.pip('check', expect_error=True)
     expected_lines = [
         "broken 1.0 requires missing, which is not installed.",
         "broken 1.0 has requirement conflict<1.0, but you have conflict 1.0.",
     ]
-    assert matches_expected_lines(result.stdout, expected_lines)
+    assert contains_expected_lines(result.stdout, expected_lines)

tests/yaml/conflict_1.yml

@@ -11,12 +11,9 @@ cases:
   response:
     - error:
         code: 0
-        stderr: ['requirement', 'is\s+incompatible']
+        stderr: ['incompatible']
   skip: old
-  # -- currently the error message is:
-  # a 1.0.0 has requirement B==1.0.0, but you'll have b 2.0.0 which is
-  # incompatible.
-  # -- better would be:
+  # -- a good error message would be:
   # A 1.0.0 has incompatible requirements B==1.0.0, B==2.0.0
 
 -

tests/yaml/conflicting_triangle.yml

@@ -14,7 +14,5 @@ cases:
       - C 1.0.0
     - error:
         code: 0
-        stderr: ['requirement c==1\.0\.0', 'is incompatible']
+        stderr: ['c==1\.0\.0', 'incompatible']
   skip: old
-  # -- currently the error message is:
-  # a 1.0.0 has requirement C==1.0.0, but you'll have c 2.0.0 which is incompatible.