Updating variables passed into threads python

I am currently having issues to do with my understanding of threading or possibly my understanding of how variables are passed/assigned thru threads in python. I have this simple program that takes in a list of current stocks that are displayed on a screen and grabs the stock information related to those. I am using threads so that I can constantly update the screen and constantly collect data. I am having two issues:

1. Inside dataCollector_thread() i understand that if i append to the stocksOnScreenListInfo that the variable (stocksOnScreenListInfo) inside main is updated.

However I don't want to append to the list but rather just reassign the list like the following but this does not work?.

def dataCollector_thread(stocksOnScreenListInfo, stocksOnScreen):
    while(True):
        placeholder = []
        for stock in stocksOnScreen:
            placeholer.append(RetrieveQuote(stock))
        stocksOnScreenListInfo = placeholder
        time.sleep(5)

2. Inside screenUpdate_thread i am wanting to update stocksOnScreen to the variable 'TSLA' defined by the function UpdateScreen. This does not seem to update its corresponding stocksOnScreen in main as when I print to check it continues to say 'AAPL'?

   def main(args): 
   
    stocksOnScreen = ["AAPL"] # List of the stocks currently displayed on LED screen
   
    stocksOnScreenListInfo = [] # The quote information list for each stock on screen 
   
    thread_data_collector = threading.Thread(target=dataCollector_thread, args=(stocksOnScreenListInfo,stocksOnScreen))
    thread_data_collector.daemon = True
    thread_data_collector.start()
   
    thread_screen = threading.Thread(target=screenUpdate_thread, args=(stocksSearchArray,stocksOnScreen))
    thread_screen.daemon = True
    thread_screen.start()
   
   
   
    def dataCollector_thread(stocksOnScreenListInfo, stocksOnScreen):
        while(True):
            for stock in stocksOnScreen:
                stocksOnScreenListInfo.append(RetrieveQuote(stock))
            time.sleep(5)
   
    def screenUpdate_thread(stocksSearchArray, stocksOnScreen):
        while(True):
            stocksOnScreen = UpdateScreen(stocksSearchArray)
   
   
    def UpdateScreen(stocksSearchArray):
        pass
   
    return ["TSLA"]
   

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