Test the report method

pkg_resources/__init__.py

@@ -317,8 +317,34 @@ class ResolutionError(Exception):
     def __repr__(self):
         return self.__class__.__name__+repr(self.args)
 
+
 class VersionConflict(ResolutionError):
-    """An already-installed version conflicts with the requested version"""
+    """
+    An already-installed version conflicts with the requested version.
+
+    Should be initialized with the installed Distribution, the requested
+    Requirement, and optionally a list of dists that required the installed
+    Distribution.
+    """
+
+    @property
+    def dist(self):
+        return self.args[0]
+
+    @property
+    def req(self):
+        return self.args[1]
+
+    @property
+    def required_by(self):
+        return self.args[2] if len(self.args) > 2 else []
+
+    def report(self):
+        template = "{self.dist} is installed but {self.req} is required"
+        if self.required_by:
+            template += " by {self.required_by}"
+        return template.format(**locals())
+
 
 class DistributionNotFound(ResolutionError):
     """A requested distribution was not found"""
@@ -763,9 +789,8 @@ class WorkingSet(object):
                 to_activate.append(dist)
             if dist not in req:
                 # Oops, the "best" so far conflicts with a dependency
-                tmpl = "%s is installed but %s is required by %s"
-                args = dist, req, list(required_by.get(req, []))
-                raise VersionConflict(tmpl % args)
+                dependent_req = list(required_by.get(req, []))
+                raise VersionConflict(dist, req, dependent_req)
 
             # push the new requirements onto the stack
             new_requirements = dist.requires(req.extras)[::-1]

pkg_resources/tests/test_resources.py

@@ -175,8 +175,8 @@ class TestDistro:
         with pytest.raises(VersionConflict) as vc:
             ws.resolve(parse_requirements("Foo==1.2\nFoo!=1.2"), ad)
 
-        msg = 'Foo 0.9 is installed but Foo==1.2 is required by []'
-        assert str(vc).endswith(msg)
+        msg = 'Foo 0.9 is installed but Foo==1.2 is required'
+        assert vc.value.report() == msg
 
     def testDistroDependsOptions(self):
         d = self.distRequires("""
@@ -218,7 +218,7 @@ class TestWorkingSet:
             ws.find(req)
 
         msg = 'Foo 1.2 is installed but Foo<1.2 is required'
-        assert str(vc).endswith(msg)
+        assert vc.value.report() == msg
 
 
 class TestEntryPoints:

setuptools/command/easy_install.py

@@ -703,10 +703,7 @@ Please make the appropriate changes for your system and try again.
                 "Could not find required distribution %s" % e.args
             )
         except VersionConflict as e:
-            raise DistutilsError(
-                "Installed distribution %s conflicts with requirement %s"
-                % e.args
-            )
+            raise DistutilsError(e.report())
         if self.always_copy or self.always_copy_from:
             # Force all the relevant distros to be copied or activated
             for dist in distros: