[Findit] Initial code of findit for crash. Add scorers to apply heuristic rules.

appengine/findit/crash/scorers/scorer.py

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Interface of scorers to score and reason Result.
+
+A Scorer represents a heuristic rule to score a culprit cl result.
+"""
+
+import logging
+
+
+class Scorer(object):  # pragma: no cover.
+
+  def GetMetric(self, result):
+    raise NotImplementedError()
+
+  def Score(self, metric):
+    """Score the result based on extracted metric."""
+    raise NotImplementedError()
+
+  def Reason(self, metric, score):
+    """Given the reason of this score."""
+    raise NotImplementedError()
+
+  def __call__(self, result):
+    """Returns score and reason of this result."""
+    metric = self.GetMetric(result)
+    if metric is None:
+      logging.warning('Cannot get needed metric of result %s for scorer %s',
+                      repr(result.ToDict()), self.name)
+      return 0, ''
+
+    score = self.Score(metric)
+    reason = self.Reason(metric, score)
+    return score, reason
+
+  @property
+  def name(self):
+    return self.__class__.__name__