def split_columns(string):
    """
    Splits the return-columns argument or reads it from .txt
    --return-columns 'temperature c, "heat$"' -> ['temperature c', '"heat$"']
    --return-columns my_vars.txt -> ['temperature c', '"heat$"']

    """
    if string.endswith('.txt'):
        with open(string, 'r') as file:
            return [col.rstrip('\n').strip() for col in file]

    return [s.strip() for s in string.split(',')]