From 107369696597c38e266263bc097608ecc3edb87d Mon Sep 17 00:00:00 2001
From: scottsanderson <ssanderson@quantopian.com>
Date: Mon, 6 Aug 2012 13:11:16 -0400
Subject: [PATCH] moving toward abstract base for event window tnfms

---
 zipline/gens/sort.py      |  2 +-
 zipline/gens/transform.py | 85 +++++++++++++++++----------------------
 2 files changed, 38 insertions(+), 49 deletions(-)

diff --git a/zipline/gens/sort.py b/zipline/gens/sort.py
index 3ff5ee3f..9755da74 100644
--- a/zipline/gens/sort.py
+++ b/zipline/gens/sort.py
@@ -27,7 +27,7 @@ def date_sort(stream_in, source_ids):
         # Incoming messages should be the output of DATASOURCE_UNFRAME.
         assert_datasource_unframe_protocol(message), \
             "Bad message in date_sort: %s" % message
-
+        
         # Only allow messages from sources we expect.
         assert message.source_id in sources, "Unexpected source: %s" % message
 
diff --git a/zipline/gens/transform.py b/zipline/gens/transform.py
index 36e15689..ee8cd649 100644
--- a/zipline/gens/transform.py
+++ b/zipline/gens/transform.py
@@ -174,20 +174,39 @@ class MovingAverage(object):
         window.update(event)
         return window.get_averages()
 
-class EventWindow(object):
-    """
-    Maintains a list of events that are within a certain timedelta
-    of the most recent tick.  The expected use of this class is to
-    track events associated with a single sid. We provide simple
-    functionality for averages, but anything more complicated
-    should be handled by a containing class.
+class EventWindow:
     """
+    Abstract base class for transform classes that calculate iterative
+    metrics on events within a given timedelta.  Maintains a list of
+    events that are within a certain timedelta of the most recent
+    tick.  Calls self.handle_add(event) for each event added to the
+    window.  Calls self.handle_remove(event) for each event removed
+    from the window.  Subclass these methods along with init(*args,
+    **kwargs) to calculate metrics over the window.  
 
-    def __init__(self, delta, fields):
+    See zipline/gens/mavg.py and zipline/gens/vwap.py for example
+    implementations of moving average and volume-weighted average
+    price.
+    """
+    # Mark this as an abstract base class.
+    __metaclass__ = ABCMeta
+
+    def __init__(self, delta, *args, **kwargs):
         self.ticks  = deque()
         self.delta  = delta
-        self.fields = fields
-        self.totals = defaultdict(float)
+        self.init(*args, **kwargs)
+        
+    @abstractmethod
+    def init(self):
+        raise NotImplementedError()
+
+    @abstractmethod
+    def handle_add(self, event):
+        raise NotImplementedError()
+
+    @abstractmethod
+    def handle_remove(self, event):
+        raise NotImplementedError()
 
     def __len__(self):
         return len(self.ticks)
@@ -196,44 +215,19 @@ class EventWindow(object):
         self.assert_well_formed(event)
         # Add new event and increment totals.
         self.ticks.append(event)
-        for field in self.fields:
-            self.totals[field] += event[field]
+        self.handle_add(event)
 
-        # We return a list of all out-of-range events we removed.
-        out_of_range = []
-
-        # Clear out expired events, decrementing totals.
+        # Clear out expired event.
+        #
         #           newest               oldest
         #             |                    |
         #             V                    V
-
-        while (self.ticks[-1].dt - self.ticks[0].dt) >= self.delta:
-            # popleft removes and returns ticks[0]
+        while (self.ticks[-1].dt - self.ticks[0].dt) > self.delta:
+            # popleft removes and returns the oldest tick in self.ticks
             popped = self.ticks.popleft()
-            # Decrement totals
-            for field in self.fields:
-                self.totals[field] -= popped[field]
-            # Add the popped element to the list of dropped events.
-            out_of_range.append(popped)
-
-        return out_of_range
-
-    def average(self, field):
-        assert field in self.fields
-        if len(self.ticks) == 0:
-            return 0.0
-        else:
-            return self.totals[field] / len(self.ticks)
-
-    def get_averages(self):
-        """
-        Return an ndict of all our tracked averages.
-        """
-        out = ndict()
-        # out.ticks = len(self.ticks)
-        for field in self.fields:
-            out[field] = self.average(field)
-        return out
+            # Subclasses should override handle_remove to define
+            # behavior for removing ticks.
+            self.handle_remove(popped)
 
     def assert_well_formed(self, event):
         assert isinstance(event, ndict), "Bad event in EventWindow:%s" % event
@@ -243,8 +237,3 @@ class EventWindow(object):
             # Something is wrong if new event is older than previous.
             assert event.dt >= self.ticks[-1].dt, \
                 "Events arrived out of order in EventWindow: %s -> %s" % (event, self.ticks[0])
-        for field in self.fields:
-            assert event.has_key(field), \
-                "Event missing [%s] in EventWindow" % field
-            assert isinstance(event[field], Number), \
-                "Got %s for %s in EventWindow" % (event[field], field)