add an assertWarns context manager for unit tests

tests/__init__.py

@@ -1,5 +1,32 @@
 import os
 import sys
+import unittest
+import warnings
 
 sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)),
                                 '../src'))
+
+class CatchWarningsMixin(object):
+    class assertWarns(object):
+        def __init__(self, warningtype, msg=''):
+            self.warningtype = warningtype
+            warnings.filterwarnings('error')
+            self.failureException = unittest.TestCase.failureException
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc_value, tb):
+            if exc_type is None:
+                try:
+                    exc_name = self.warningtype.__name__
+                except AttributeError:
+                    exc_name = str(self.warningtype)
+                raise self.failureException(
+                    "{0} not raised".format(exc_name))
+
+            if not issubclass(exc_type, self.warningtype):
+                raise self.failureException('"%s" does not match "%s"' %
+                     (self.warningtype.__name__, str(exc_type.__name__)))
+
+            return True