Monkeypatch unittest on function call, not on load

Doing changes like these on module load is very intransparent and the
explicit call of the function telegraphs this way better.

Inspired by requests:
https://github.com/kennethreitz/requests/blob/9bbab338fdbb562b923ba2d8a80f0bfba697fa41/requests/__init__.py#L53-L54

tests/test.py

@@ -24,12 +24,15 @@ if sys.version_info >= (3,):
     from urllib.request import urlopen
     from urllib.error import HTTPError
     from urllib.parse import urljoin
+
     generator_method_type = 'method'
     str_cls = str
 else:
-    from . import unittest_compat  # NOQA (monkeypatches the unittest module)
+    from . import unittest_compat
     from urlparse import urljoin
     from urllib2 import HTTPError, urlopen
+
+    unittest_compat.inject_into_unittest()
     generator_method_type = 'instancemethod'
     str_cls = unicode  # NOQA (obviously undefined in Py3)
 

tests/unittest_compat.py

@@ -3,7 +3,8 @@ import re
 import unittest
 
 
-if sys.version_info < (2, 7):
+def inject_into_unittest():
+    if sys.version_info < (2, 7):
         def assertIn(self, member, container, msg=None):
             """Just like self.assertTrue(a in b), but with a nicer default message."""
             if member not in container:
@@ -62,6 +63,6 @@ if sys.version_info < (2, 7):
                 self.fail(msg)
         unittest.TestCase.assertIsInstance = assertIsInstance
 
-else:
+    else:
         unittest.TestCase.assertRegex = unittest.TestCase.assertRegexpMatches
         unittest.TestCase.assertNotRegex = unittest.TestCase.assertNotRegexpMatches