nswamy commented on a change in pull request #14685: [Fit API] improve event 
handlers
URL: https://github.com/apache/incubator-mxnet/pull/14685#discussion_r277221859
 
 

 ##########
 File path: python/mxnet/gluon/contrib/estimator/event_handler.py
 ##########
 @@ -16,85 +16,169 @@
 # under the License.
 
 # coding: utf-8
-# pylint: disable=wildcard-import
+# pylint: disable=wildcard-import, unused-argument
 """Gluon EventHandlers for Estimators"""
 
-__all__ = ['EventHandler', 'LoggingHandler']
 import logging
 import os
 import time
 import warnings
 
 import numpy as np
 
+from ....metric import EvalMetric, Loss
 
-class EventHandler(object):
-    """Basic for event handlers
 
-        :py:class:`EventHandler` can perform user defined functions at
-        different stages of training: train begin, epoch begin, batch begin,
-        batch end, epoch end, train end.
-
-        Parameters
-        ----------
-        estimator : Estimator
-            The :py:class:`Estimator` to get training statistics
-        """
+class TrainBegin(object):
+    def train_begin(self, estimator, *args, **kwargs):
+        pass
 
-    def __init__(self):
-        self._estimator = None
 
-    @property
-    def estimator(self):
-        return self._estimator
+class TrainEnd(object):
+    def train_end(self, estimator, *args, **kwargs):
+        pass
 
-    @estimator.setter
-    def estimator(self, estimator):
-        self._estimator = estimator
 
-    def train_begin(self):
+class EpochBegin(object):
+    def epoch_begin(self, estimator, *args, **kwargs):
         pass
 
-    def train_end(self):
-        pass
 
-    def batch_begin(self):
-        pass
+class EpochEnd(object):
+    def epoch_end(self, estimator, *args, **kwargs):
+        return False
 
-    def batch_end(self):
-        pass
 
-    def epoch_begin(self):
+class BatchBegin(object):
+    def batch_begin(self, estimator, *args, **kwargs):
         pass
 
-    def epoch_end(self):
-        pass
 
+class BatchEnd(object):
+    def batch_end(self, estimator, *args, **kwargs):
+        return False
+
+
+class MetricHandler(EpochBegin, BatchEnd):
+    """Metric Handler that update metric values at batch end
+
+    :py:class:`MetricHandler` takes model predictions and true labels
+    and update the metrics, it also update metric wrapper for loss with loss 
values
+    Validation loss and metrics will be handled by 
:py:class:`ValidationHandler`
+
+    Parameters
+    ----------
+    train_metrics : List of EvalMetrics
+        training metrics to be updated at batch end
+    """
+
+    def __init__(self, train_metrics):
+        self.train_metrics = train_metrics or []
+        # order to be called among all callbacks
+        # metrics need to be calculated before other callbacks can access them
+        self.priority = -np.Inf
 
 Review comment:
   lets call this out explicitly in the documentation.

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