Counter: expose the previous and changed values.

This commit extends the `Counter` API with the `pre_value` and
`post_value` attributes. These attributes are set when a counter
is changed. Previously, if the counter was modified concurrently,
it was not possible to access the exact value to which the counter
was set after a successful increment or decrement.

Author: Lukasz Kawczynski

kazoo/recipe/counter.py

@@ -34,10 +34,14 @@ class Counter(object):
         counter += 2
         counter -= 1
         counter.value == 1
+        counter.pre_value == 2
+        counter.post_value == 1
 
         counter = zk.Counter("/float", default=1.0)
         counter += 2.0
         counter.value == 3.0
+        counter.pre_value == 1.0
+        counter.post_value == 3.0
 
     """
     def __init__(self, client, path, default=0):
@@ -53,6 +57,8 @@ def __init__(self, client, path, default=0):
         self.default = default
         self.default_type = type(default)
         self._ensured_path = False
+        self.pre_value = None
+        self.post_value = None
 
     def _ensure_node(self):
         if not self._ensured_path:
@@ -79,12 +85,15 @@ def _change(self, value):
         return self
 
     def _inner_change(self, value):
-        data, version = self._value()
-        data = repr(data + value).encode('ascii')
+        self.pre_value, version = self._value()
+        post_value = self.pre_value + value
+        data = repr(post_value).encode('ascii')
         try:
             self.client.set(self.path, data, version=version)
         except BadVersionError:  # pragma: nocover
+            self.post_value = None
             raise ForceRetryError()
+        self.post_value = post_value
 
     def __add__(self, value):
         """Add value to counter."""

kazoo/tests/test_counter.py

@@ -33,3 +33,15 @@ def test_errors(self):
         counter = self._makeOne()
         self.assertRaises(TypeError, counter.__add__, 2.1)
         self.assertRaises(TypeError, counter.__add__, b"a")
+
+    def test_pre_post_values(self):
+        counter = self._makeOne()
+        eq_(counter.value, 0)
+        eq_(counter.pre_value, None)
+        eq_(counter.post_value, None)
+        counter += 2
+        eq_(counter.pre_value, 0)
+        eq_(counter.post_value, 2)
+        counter -= 3
+        eq_(counter.pre_value, 2)
+        eq_(counter.post_value, -1)