Error "PipeMapRed.waitOutputThreads(): subprocess failed with code 1" when accessing a list of lists by index on hadoop for mapreduce python program

I wrote a mapreduce program to resolve matrix operation "X-MN" where M,N,X are matrices with integer values. In order to do that I need to have a list of lists. For instance:

M=[[1,2,3],[4,5,6],[7,8,9]] which represents a matrix of 3×3.

During some tshoot I found the following:

test=[1,2]
test2=[4,5]
test.append(test2) #this works
a=test[0]
b=test[1]
c=test[2]   #this also works
print(c)   
output => [4,5]

So, until this point everyting seems to be fine.

However, from my actual mapper.py program code below.

#!/usr/bin/env python3

import sys

row_m, col_m = map(int, sys.argv[1:])
row_n = col_m
col_n = row_m
all_matrices=list()

for line in sys.stdin:
    all_matrices.append(line)

M_values=list()
N_values=list()
X_values = list()

for line in all_matrices:
    elements=line.rstrip().split(", ")
    index = elements[0]
    row_index = elements[1]
    row_values = elements[2:]
    if index == "1":
        M_values.append(row_values)
    elif index == "2":
        N_values.append(row_values)
    elif index == "3":
        X_values.append(row_values)
print(N_values) #this works

print(N_values[0]) #this exact command does not work and causes below errors
                   # and I need it in order to my logic works
                   #without "N_values[0]" command, above code works just fine.

Basically, until this point I have only read from input mapper data on stdin, store it on 3 different lists, and when I try to recover a matrix row (an element list) it fails.

Here is my run.sh file that executes my mapper python program.

#!/bin/bash

arg1=2
arg2=2
hadoop jar ~/hadoop-streaming-3.1.4.jar 
-D mapred.reduce.tasks=0 
-file ./mapper.py 
-mapper "./mapper.py $arg1 $arg2" 
-input /matrices 
-output /output

matrices folder contains 3 txt files for each matrix. This is my input mapper data and I am sure I can gather it as whithout the problematic command I am able to see the data on "N_values"

Here are the errors:

22/09/15 18:14:13 INFO mapreduce.Job: Running job: job_1663260914490_0011
22/09/15 18:14:21 INFO mapreduce.Job: Job job_1663260914490_0011 running in uber mode : false
22/09/15 18:14:21 INFO mapreduce.Job:  map 0% reduce 0%
22/09/15 18:14:32 INFO mapreduce.Job: Task Id : attempt_1663260914490_0011_m_000001_0, Status : FAILED
Error: java.lang.RuntimeException: PipeMapRed.waitOutputThreads(): subprocess failed with code 1
        at org.apache.hadoop.streaming.PipeMapRed.waitOutputThreads(PipeMapRed.java:325)
        at org.apache.hadoop.streaming.PipeMapRed.mapRedFinished(PipeMapRed.java:538)
        at org.apache.hadoop.streaming.PipeMapper.close(PipeMapper.java:130)
        at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:61)
        at org.apache.hadoop.streaming.PipeMapRunner.run(PipeMapRunner.java:34)
        at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:461)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:344)
        at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:177)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:422)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1926)
        at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:171)

.
.
.
22/09/15 18:15:14 ERROR streaming.StreamJob: Job not successful!
Streaming Command Failed!

Additionally, I execute my program on hadoop server to assure that it is not a syntaxis issue and it actually works. Test as follow:
Matrix N_values was [[7,9],[4,2]]

[xxxx]$ cat matrix.txt | python mapper.py 2 2
[['7', '9'], ['4', '2']]     
[4, 5]
['7', '9']

Then I do not think this is a problem with my python code. Maybe it is an issue with the commands on run.sh file, as I am a newbie working with hadoop, or the .jar version I am using.

I hope someone can help me to resolve this problem. It would be much appreciated.