Merge pull request #2290 from dstufft/distribute-hack-fix

Check if distribute is installed before checking installed version
(cherry picked from commit 12e6d47264)

CHANGES.txt

@@ -10,6 +10,9 @@
 * Fix an issue where using -v with dependency links would cause an exception
   saying ``TypeError: 'InstallationCandidate' object is not iterable``.
 
+* Fix an issue where upgrading distribute would cause an exception saying
+  ``TypeError: expected string or buffer``.
+
 
 **6.0.3 (2014-12-23)**
 

pip/req/req_set.py

@@ -494,6 +494,7 @@ class RequirementSet(object):
         distribute_req = pkg_resources.Requirement.parse("distribute>=0.7")
         for req in to_install:
             if (req.name == 'distribute'
+                    and req.installed_version is not None
                     and req.installed_version in distribute_req):
                 to_install.remove(req)
                 to_install.append(req)