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Scikit Machine Learning Tutorial Part 8 Question

import pandas as pd

    for each_dir in stock_list[1:25]:
        each_file = os.listdir(each_dir)
        ticker = each_dir.split("")[1]
        ticker_list.append(ticker)

        starting_stock_value = False
        starting_sp500_value = False
        
        if len(each_file) > 0:
            for file in each_file:
            .....
                    if not starting_stock_value:
                        starting_stock_value = stock_price
                    if not starting_sp500_value:
                        starting_sp500_value = sp500_value
             .....

Question:

1.  I'm having some trouble understanding the portion of the (abbreviated) code above.  As I understand it, you set starting_stock_value and starting_sp500_value to 'False' before you enter the loops where data is processed.  I get conceptually that we want the earliest price for each security to be our 'base' and that there is no change (since there is no prior price).  What I don't understand is how 'if not ...' is working.  We initially set starting_stock_value and starting_sp500_value to 'False'.  When we get down to 'if not...' on the first run the value is 'True' ('if not "False") so we set starting_stock_value to the initial stock price.   When we loop through each file for a particular stock, does the 'if not' statement no longer execute because starting_stock_value and starting_sp500_value are not boolean (floats) and aren't reset to a boolean until the next security (when they are set back to false)?

Thanks in advance.