Change when we warn about dependency conflicts during pip install

src/pip/_internal/commands/install.py

@@ -40,6 +40,7 @@
     from typing import Iterable, List, Optional
 
     from pip._internal.models.format_control import FormatControl
+    from pip._internal.operations.check import ConflictDetails
     from pip._internal.req.req_install import InstallRequirement
     from pip._internal.wheel_builder import BinaryAllowedPredicate
 
@@ -371,13 +372,14 @@ def run(self, options, args):
                 requirement_set
             )
 
-            # Consistency Checking of the package set we're installing.
+            # Check for conflicts in the package set we're installing.
+            conflicts = None  # type: Optional[ConflictDetails]
             should_warn_about_conflicts = (
                 not options.ignore_dependencies and
                 options.warn_about_conflicts
             )
             if should_warn_about_conflicts:
-                self._warn_about_conflicts(to_install)
+                conflicts = self._determine_conflicts(to_install)
 
             # Don't warn about script install locations if
             # --target has been specified
@@ -419,6 +421,10 @@ def run(self, options, args):
                 except Exception:
                     pass
                 items.append(item)
+
+            if conflicts is not None:
+                self._warn_about_conflicts(conflicts)
+
             installed_desc = ' '.join(items)
             if installed_desc:
                 write_output(
@@ -498,16 +504,23 @@ def _handle_target_dir(self, target_dir, target_temp_dir, upgrade):
                     target_item_dir
                 )
 
-    def _warn_about_conflicts(self, to_install):
-        # type: (List[InstallRequirement]) -> None
+    def _determine_conflicts(self, to_install):
+        # type: (List[InstallRequirement]) -> Optional[ConflictDetails]
         try:
-            package_set, _dep_info = check_install_conflicts(to_install)
+            return check_install_conflicts(to_install)
         except Exception:
-            logger.error("Error checking for conflicts.", exc_info=True)
-            return
-        missing, conflicting = _dep_info
+            logger.error(
+                "Error while checking for conflicts. Please file an issue on "
+                "pip's issue tracker: https://github.com/pypa/pip/issues/new",
+                exc_info=True
+            )
+            return None
+
+    def _warn_about_conflicts(self, conflict_details):
+        # type: (ConflictDetails) -> None
+        package_set, (missing, conflicting) = conflict_details
 
-        # NOTE: There is some duplication here from pip check
+        # 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]:

src/pip/_internal/operations/check.py

@@ -29,6 +29,7 @@
     MissingDict = Dict[str, List[Missing]]
     ConflictingDict = Dict[str, List[Conflicting]]
     CheckResult = Tuple[MissingDict, ConflictingDict]
+    ConflictDetails = Tuple[PackageSet, CheckResult]
 
 PackageDetails = namedtuple('PackageDetails', ['version', 'requires'])
 
@@ -99,7 +100,7 @@ def check_package_set(package_set, should_ignore=None):
 
 
 def check_install_conflicts(to_install):
-    # type: (List[InstallRequirement]) -> Tuple[PackageSet, CheckResult]
+    # type: (List[InstallRequirement]) -> ConflictDetails
     """For checking if the dependency graph would be consistent after \
     installing given requirements
     """