Python 'str' object does not support item assignment

I'm developing a python script for an embedded system that sends data to ThingSpeak.

To do this (and for studing python) I used the thread module and also the ThingSpeak module is used.

Basically my script create 2 threads:

• One for sending data to ThingSpeak every 10 seconds.
• One to toggle a LED.

Everything work except the publish on ThingSpeak. Note: the publish on thingSpeak without the thread works.

I get this error: "TypeError: 'str' object does not support item assignment"

Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "thingSpeak.py", line 57, in ThingSpeak_Thread
    response = thingSpeakHandler.update(jsonMessage)
  File "/usr/lib/python2.7/site-packages/thingspeak/thingspeak.py", line 110, in update
    data['api_key'] = self.write_key
TypeError: 'str' object does not support item assignment

I think that this error is related to the addition of threads.

This is my code:

import onionGpio
import time
import thingspeak
import sys
import threading
import random
import thread
import json

# Declare variables
heartBeatDelaySec   = 1.0
heartBeatLedStatus  = 0
heartBeatDisable    = False

thingSpeakDelaySec  = 10

thingSpeakChannel = 98765
thingSpeakWriteKey = '1234556'
thingSpeakReadKey = '123456'


def HeartBeatThread( name, delay, runEvent ):
  global heartBeatLedStatus

  print ("Task " + name + " started")

  while runEvent.is_set():
    blueLed.setValue( heartBeatLedStatus )

    if( heartBeatLedStatus == 0 ):
      heartBeatLedStatus = 1
    else:
      heartBeatLedStatus = 0

    time.sleep(delay)

  # Shut-down Led
  blueLed.setValue( 1 )


def ThingSpeak_Thread( name, delay, thingSpeakChannel, thingSpeakWriteKey, runEvent ):

  thingSpeakHandler = thingspeak.Channel( id=thingSpeakChannel, write_key=thingSpeakWriteKey, fmt='json' )

  while runEvent.is_set():
    temp = random.randint(12, 38)
    rH   = random.randint(30, 68)

    # Build JSON with data
    data = {}
    data['1'] = temp
    data['2'] = rH
    jsonMessage = json.dumps(data)

    print( "Publishing: " + jsonMessage )

    response = thingSpeakHandler.update(jsonMessage)

    time.sleep(delay)


# Led Initialize
blueLed   = onionGpio.OnionGpio(15)
greenLed  = onionGpio.OnionGpio(16)
redLed    = onionGpio.OnionGpio(17)

blueLed.setOutputDirection(1)
greenLed.setOutputDirection(1)
redLed.setOutputDirection(1)

blueLed.setValue(1)
greenLed.setValue(1)
redLed.setValue(1)
print ("GPIO configured")


# Initialize Threads
runEvent = threading.Event()
runEvent.set()

heartBeatThreadHandler = threading.Thread(target = HeartBeatThread, args = ("Heart Beat", heartBeatDelaySec, runEvent))

thingSpeakThreadHandler = threading.Thread(target = ThingSpeak_Thread, args = ("ThingSpeak", thingSpeakDelaySec, thingSpeakChannel, thingSpeakWriteKey, runEvent))

# Start Threads
heartBeatThreadHandler.start()
thingSpeakThreadHandler.start()

try:
  while( True ):
    time.sleep(.01)
except:
  print ("Attempting to close threads. Max wait = " + str(max(heartBeatDelaySec, thingSpeakDelaySec)) + "sec")
  runEvent.clear()
  heartBeatThreadHandler.join()
  thingSpeakThreadHandler.join()
  print ("Threads successfully closed")

Thanks for the help in advance.

Best Regards, Federico