add with support (for testing), and add global instance, and handle it

casimport/__init__.py

@@ -64,8 +64,17 @@ class CASFinder(MetaPathFinder, Loader):
 	def __init__(self):
 		self._loaders = []
 
+		if [ x for x in sys.meta_path if isinstance(x, self.__class__) ]:
+			raise RuntimeError('cannot register more than on CASFinder')
+
 		sys.meta_path.append(self)
 
+	def __enter__(self):
+		return self
+
+	def __exit__(self, exc_type, exc_value, traceback):
+		self.disconnect()
+
 	def disconnect(self):
 		try:
 			sys.meta_path.remove(self)
@@ -100,9 +109,19 @@ class CASFinder(MetaPathFinder, Loader):
 			hash, load = module.__spec__.loader_state
 			load.exec_module(hash, module)
 
+# The global version
+_casfinder = CASFinder()
+
 import unittest
 
 class Test(unittest.TestCase):
+	def setUp(self):
+		self.old_meta_path = sys.meta_path
+		sys.meta_path = [ x for x in sys.meta_path if not isinstance(x, CASFinder) ]
+
+	def tearDown(self):
+		sys.meta_path = self.old_meta_path
+
 	def test_casimport(self):
 		# That a CASFinder
 		f = CASFinder()
@@ -125,3 +144,9 @@ class Test(unittest.TestCase):
 
 		# and when disconnected as second time, nothing happens
 		f.disconnect()
+
+	def test_multiplecas(self):
+		# that once we have one
+		with CASFinder() as f:
+			# if we try to create a second, it fails
+			self.assertRaises(RuntimeError, CASFinder)