Add class ObservableMixin

This new class will be used to observe iterables over elements read
in the worker, so that we can size them.
----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119882940

Author: gildea

google/cloud/dataflow/coders/observable.py

@@ -0,0 +1,33 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+"""Observable base class for iterables."""
+
+
+class ObservableMixin(object):
+  """An observable iterable.
+
+  Subclasses need to call self.notify_observers with any object yielded.
+  """
+
+  def __init__(self):
+    self.observers = []
+
+  def register_observer(self, callback):
+    self.observers.append(callback)
+
+  def notify_observers(self, value, **kwargs):
+    for o in self.observers:
+      o(value, **kwargs)

google/cloud/dataflow/coders/observable_test.py

@@ -0,0 +1,54 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for the Observable mixin class."""
+
+import logging
+import unittest
+
+
+from google.cloud.dataflow.coders import observable
+
+
+class ObservableMixinTest(unittest.TestCase):
+  observed_count = 0
+  observed_sum = 0
+  observed_keys = []
+
+  def observer(self, value, key=None):
+    self.observed_count += 1
+    self.observed_sum += value
+    self.observed_keys.append(key)
+
+  def test_observable(self):
+    class Watched(observable.ObservableMixin):
+
+      def __iter__(self):
+        for i in (1, 4, 3):
+          self.notify_observers(i, key='a%d' % i)
+          yield i
+
+    watched = Watched()
+    watched.register_observer(lambda v, key: self.observer(v, key=key))
+    for _ in watched:
+      pass
+
+    self.assertEquals(3, self.observed_count)
+    self.assertEquals(8, self.observed_sum)
+    self.assertEquals(['a1', 'a3', 'a4'], sorted(self.observed_keys))
+
+
+if __name__ == '__main__':
+  logging.getLogger().setLevel(logging.INFO)
+  unittest.main()