diff --git a/python/setup.py b/python/setup.py
index 23193cba..f7ee6357 100755
--- a/python/setup.py
+++ b/python/setup.py
@@ -146,6 +146,7 @@ if __name__ == '__main__':
     )
     os.environ['PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION'] = 'cpp'
 
+  # Keep this list of dependencies in sync with tox.ini.
   install_requires = ['six', 'setuptools']
   if sys.version_info <= (2,7):
     install_requires.append('ordereddict')
diff --git a/python/tox.ini b/python/tox.ini
index ebe75459..d758512d 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -6,6 +6,7 @@ envlist =
     py{26,27}-{cpp,python}
 
 [testenv]
+usedevelop=true
 setenv =
     cpp: LD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
@@ -16,6 +17,7 @@ commands =
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
 deps =
+    # Keep this list of dependencies in sync with setup.py.
     six
     py26: ordereddict
     py26: unittest2