[SPARK-2954] Fix MLlib _deserialize_double on Python 2.6.

Author: JoshRosen

python/pyspark/mllib/_common.py

@@ -16,6 +16,7 @@
 #
 
 import struct
+import sys
 import numpy
 from numpy import ndarray, float64, int64, int32, array_equal, array
 from pyspark import SparkContext, RDD
@@ -78,6 +79,14 @@
 LABELED_POINT_MAGIC = 4
 
 
+# Workaround for SPARK-2954: before Python 2.7, struct.unpack couldn't unpack bytearray()s.
+if sys.version_info[:2] <= (2, 6):
+    def _unpack(fmt, string):
+        return struct.unpack(fmt, buffer(string))
+else:
+    _unpack = struct.unpack
+
+
 def _deserialize_numpy_array(shape, ba, offset, dtype=float64):
     """
     Deserialize a numpy array of the given type from an offset in
@@ -191,7 +200,7 @@ def _deserialize_double(ba, offset=0):
         raise TypeError("_deserialize_double called on a %s; wanted bytearray" % type(ba))
     if len(ba) - offset != 8:
         raise TypeError("_deserialize_double called on a %d-byte array; wanted 8 bytes." % nb)
-    return struct.unpack("d", ba[offset:])[0]
+    return _unpack("d", ba[offset:])[0]
 
 
 def _deserialize_double_vector(ba, offset=0):