[SPARK-972] Added detailed callsite info for ValueError in context.py…

… (resubmitted)

Author: jyotiska <jyotiska123@gmail.com>

Closes #34 from jyotiska/pyspark_code and squashes the following commits:

c9439be [jyotiska] replaced dict with namedtuple
a6bf4cd [jyotiska] added callsite info for context.py

python/pyspark/context.py

@@ -20,6 +20,7 @@
 import sys
 from threading import Lock
 from tempfile import NamedTemporaryFile
+from collections import namedtuple
 
 from pyspark import accumulators
 from pyspark.accumulators import Accumulator
@@ -29,6 +30,7 @@
 from pyspark.java_gateway import launch_gateway
 from pyspark.serializers import PickleSerializer, BatchedSerializer, UTF8Deserializer
 from pyspark.storagelevel import StorageLevel
+from pyspark import rdd
 from pyspark.rdd import RDD
 
 from py4j.java_collections import ListConverter
@@ -83,6 +85,11 @@ def __init__(self, master=None, appName=None, sparkHome=None, pyFiles=None,
             ...
         ValueError:...
         """
+        if rdd._extract_concise_traceback() is not None:
+            self._callsite = rdd._extract_concise_traceback()
+        else:
+            tempNamedTuple = namedtuple("Callsite", "function file linenum")
+            self._callsite = tempNamedTuple(function=None, file=None, linenum=None)
         SparkContext._ensure_initialized(self, gateway=gateway)
 
         self.environment = environment or {}
@@ -169,7 +176,14 @@ def _ensure_initialized(cls, instance=None, gateway=None):
 
             if instance:
                 if SparkContext._active_spark_context and SparkContext._active_spark_context != instance:
-                    raise ValueError("Cannot run multiple SparkContexts at once")
+                    currentMaster = SparkContext._active_spark_context.master
+                    currentAppName = SparkContext._active_spark_context.appName
+                    callsite = SparkContext._active_spark_context._callsite
+
+                    # Raise error if there is already a running Spark context
+                    raise ValueError("Cannot run multiple SparkContexts at once; existing SparkContext(app=%s, master=%s)" \
+                        " created by %s at %s:%s " \
+                        % (currentAppName, currentMaster, callsite.function, callsite.file, callsite.linenum))
                 else:
                     SparkContext._active_spark_context = instance
 

python/pyspark/rdd.py

@@ -18,6 +18,7 @@
 from base64 import standard_b64encode as b64enc
 import copy
 from collections import defaultdict
+from collections import namedtuple
 from itertools import chain, ifilter, imap
 import operator
 import os
@@ -42,12 +43,14 @@
 __all__ = ["RDD"]
 
 def _extract_concise_traceback():
+    """
+    This function returns the traceback info for a callsite, returns a dict
+    with function name, file name and line number
+    """
     tb = traceback.extract_stack()
+    callsite = namedtuple("Callsite", "function file linenum")
     if len(tb) == 0:
-        return "I'm lost!"
-    # HACK:  This function is in a file called 'rdd.py' in the top level of
-    # everything PySpark.  Just trim off the directory name and assume
-    # everything in that tree is PySpark guts.
+        return None
     file, line, module, what = tb[len(tb) - 1]
     sparkpath = os.path.dirname(file)
     first_spark_frame = len(tb) - 1
@@ -58,16 +61,20 @@ def _extract_concise_traceback():
             break
     if first_spark_frame == 0:
         file, line, fun, what = tb[0]
-        return "%s at %s:%d" % (fun, file, line)
+        return callsite(function=fun, file=file, linenum=line)
     sfile, sline, sfun, swhat = tb[first_spark_frame]
     ufile, uline, ufun, uwhat = tb[first_spark_frame-1]
-    return "%s at %s:%d" % (sfun, ufile, uline)
+    return callsite(function=sfun, file=ufile, linenum=uline)
 
 _spark_stack_depth = 0
 
 class _JavaStackTrace(object):
     def __init__(self, sc):
-        self._traceback = _extract_concise_traceback()
+        tb = _extract_concise_traceback()
+        if tb is not None:
+            self._traceback = "%s at %s:%s" % (tb.function, tb.file, tb.linenum)
+        else:
+            self._traceback = "Error! Could not extract traceback info"
         self._context = sc
 
     def __enter__(self):