Python UDF Error

hi all,

My script basically performs a calculation and outputs an array of values.

Pig script:

register pcent.py using jython as pc;
data = load 'Data.csv' as (value:int);
B = FOREACH data GENERATE pc.ptile(value);
C = group B by (percentiles);
store C into 'C';

UDF:

@outputSchema("y:float")
def ptile(value):
    unique = set(value)
    maps = {}
    pc = float(1)/(len(unique)-1)
    for n, i in enumerate(unique):
        maps[i] = (n*pc)
    return [maps.get(i) for i in value]

The python script runs fine but I keep getting an error in Pig and the stack trace is not very helpful:

Backend error message

org.apache.pig.backend.executionengine.ExecException: ERROR 0: Error executing function
    at org.apache.pig.scripting.jython.JythonFunction.exec(JythonFunction.java:120)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNext(POUserFunc.java:337)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNext(POUserFunc.java:410)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.getNext(PhysicalOperator.java:344)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.processPlan(POForEach.java:372)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.getNext(POForEach.java:297)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.runPipeline(PigGenericMapBase.java:283)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.map(PigGenericMapBase.java:278)
    at o

Pig Stack Trace

ERROR 0: Error executing function

org.apache.pig.backend.executionengine.ExecException: ERROR 0: Error executing function
    at org.apache.pig.scripting.jython.JythonFunction.exec(JythonFunction.java:120)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNext(POUserFunc.java:337)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNext(POUserFunc.java:410)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.getNext(PhysicalOperator.java:344)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.processPlan(POForEach.java:372)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.getNext(POForEach.java:297)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.runPipeline(PigGenericMapBase.java:283)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.map(PigGenericMapBase.java:278)
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