test_free_port: monkey patching an object shouldn't impact other class/test/module/...

setup.py

@@ -214,5 +214,8 @@ setup(name=name,
         ],
       },
       test_suite='slapos.test',
-      tests_require=[ 'jsonschema' ],
+      tests_require=[
+        'jsonschema',
+        'mock',
+      ],
     )

slapos/test/recipe/test_free_port.py

 import socket
+import sys
 import unittest
 
+from mock import patch
+
 from slapos.recipe import free_port
 
 class SocketMock():
@@ -14,11 +17,14 @@ class SocketMock():
 
   bind = close = nothing_happen
 
-import sys
-sys.modules['socket'].socket = SocketMock
+def useMock(function):
+  def withMock(function):
+    with patch('slapos.recipe.free_port.socket.socket', new=SocketMock):
+      return function
+  return withMock
 
 class FreePortTest(unittest.TestCase):
-  def afterSetup(self):
+  def setUp(self):
     SocketMock.bind = SocketMock.close = SocketMock.nothing_happen
 
   def new_recipe(self, **kw):
@@ -48,10 +54,12 @@ class FreePortTest(unittest.TestCase):
     options.update(kw)
     return free_port.Recipe(buildout=buildout, name='free_port', options=options)
 
+  @useMock
   def test_ifNoBusyPortThenMinPortIsAlwaysReturned(self):
     recipe = self.new_recipe(minimum=2000)
     self.assertEqual(recipe.options['port'], '2000')
 
+  @useMock
   def test_iterateUntilFreePortIsFound(self):
     def bindFailExceptOnPort2020(socket_instance, binding):
       ip, port = binding
@@ -61,6 +69,7 @@ class FreePortTest(unittest.TestCase):
     recipe = self.new_recipe(minimum=2000)
     self.assertEqual(recipe.options['port'], '2020')
 
+  @useMock
   def test_returnsPort0IfNoPortIsFreeInRange(self):
     def bindAlwaysFail(socket_instance, binding):
       raise socket.error()