Added test to verify _psycopg can be imported

NEWS

@@ -24,6 +24,8 @@ What's new in psycopg 2.5.4
 - Don't ignore silently the `cursor.callproc` argument without a length.
 - Added a few errors missing from `~psycopg2.errorcodes`, defined by
   PostgreSQL but not documented.
+- Make sure the internal `_psycopg.so` module can be imported stand-alone
+  (to allow modules juggling such as the one described in :ticket:`#201`).
 
 
 What's new in psycopg 2.5.3

tests/test_module.py

@@ -22,8 +22,12 @@
 # FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 # License for more details.
 
+import os
+import sys
+from subprocess import Popen
+
 from testutils import unittest, skip_before_python, skip_before_postgres
-from testutils import ConnectingTestCase, skip_copy_if_green
+from testutils import ConnectingTestCase, skip_copy_if_green, script_to_py3
 
 import psycopg2
 
@@ -295,6 +299,27 @@ class ExceptionsTestCase(ConnectingTestCase):
         self.assert_(e1.cursor is None)
 
 
+class TestExtensionModule(unittest.TestCase):
+    def test_import_internal(self):
+        # check that the internal package can be imported "naked"
+        # we may break this property if there is a compelling reason to do so,
+        # however having it allows for some import juggling such as the one
+        # required in ticket #201.
+        pkgdir = os.path.dirname(psycopg2.__file__)
+        pardir = os.path.dirname(pkgdir)
+        self.assert_(pardir in sys.path)
+        script = ("""
+import sys
+sys.path.remove(%r)
+sys.path.insert(0, %r)
+import _psycopg
+""" % (pardir, pkgdir))
+
+        proc = Popen([sys.executable, '-c', script_to_py3(script)])
+        proc.communicate()
+        self.assertEqual(0, proc.returncode)
+
+
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)