Make AccumulatorParam an abstract base class.

python/pyspark/accumulators.py

@@ -61,6 +61,7 @@ Traceback (most recent call last):
 Exception:...
 """
 
+from abc import ABCMeta, abstractmethod
 import struct
 import SocketServer
 import threading
@@ -90,8 +91,7 @@ class Accumulator(object):
 
     While C{SparkContext} supports accumulators for primitive data types like C{int} and
     C{float}, users can also define accumulators for custom types by providing a custom
-    C{AccumulatorParam} object with a C{zero} and C{addInPlace} method. Refer to the doctest
-    of this module for an example.
+    L{AccumulatorParam} object. Refer to the doctest of this module for an example.
     """
 
     def __init__(self, aid, value, accum_param):
@@ -134,7 +134,30 @@ class Accumulator(object):
         return "Accumulator<id=%i, value=%s>" % (self.aid, self._value)
 
 
-class AddingAccumulatorParam(object):
+class AccumulatorParam(object):
+    """
+    Helper object that defines how to accumulate values of a given type.
+    """
+    __metaclass__ = ABCMeta
+
+    @abstractmethod
+    def zero(self, value):
+        """
+        Provide a "zero value" for the type, compatible in dimensions with the
+        provided C{value} (e.g., a zero vector)
+        """
+        return
+
+    @abstractmethod
+    def addInPlace(self, value1, value2):
+        """
+        Add two values of the accumulator's data type, returning a new value;
+        for efficiency, can also update C{value1} in place and return it.
+        """
+        return
+
+
+class AddingAccumulatorParam(AccumulatorParam):
     """
     An AccumulatorParam that uses the + operators to add values. Designed for simple types
     such as integers, floats, and lists. Requires the zero value for the underlying type

python/pyspark/context.py

@@ -148,16 +148,11 @@ class SparkContext(object):
 
     def accumulator(self, value, accum_param=None):
         """
-        Create an C{Accumulator} with the given initial value, using a given
-        AccumulatorParam helper object to define how to add values of the data
-        type if provided. Default AccumulatorParams are used for integers and
-        floating-point numbers if you do not provide one. For other types, the
-        AccumulatorParam must implement two methods:
-        - C{zero(value)}: provide a "zero value" for the type, compatible in
-          dimensions with the provided C{value} (e.g., a zero vector).
-        - C{addInPlace(val1, val2)}: add two values of the accumulator's data
-          type, returning a new value; for efficiency, can also update C{val1}
-          in place and return it.
+        Create an L{Accumulator} with the given initial value, using a given
+        L{AccumulatorParam} helper object to define how to add values of the
+        data type if provided. Default AccumulatorParams are used for integers
+        and floating-point numbers if you do not provide one. For other types,
+        a custom AccumulatorParam can be used.
         """
         if accum_param == None:
             if isinstance(value, int):