Add a Stub testing helper.

tests/helpers/stub.py

@@ -0,0 +1,25 @@
+
+
+class Stub(object):
+    """A testing double that tracks calls."""
+
+    def __init__(self):
+        self.calls = []
+        self._exceptions = []
+
+    def set_exceptions(self, *exceptions):
+        self._exceptions = list(exceptions)
+
+    def add_call(self, name, *args, **kwargs):
+        self.add_call_exact(name, args, kwargs)
+
+    def add_call_exact(self, name, args, kwargs):
+        self.calls.append((name, args, kwargs))
+
+    def maybe_raise(self):
+        if not self._exceptions:
+            return
+        exc = self._exceptions.pop(0)
+        if exc is None:
+            return
+        raise exc