def subject(source:str, start_index:int, symbol_count:int=None)-> str:
    if start_index > len(source):
        raise ValueError('start_index must be smaller than the length of source')
    if symbol_count is None:
        symbol_count = len(source)-start_index
    if start_index+symbol_count > len(source):
        raise ValueError('start_index must be smaller than the length of source')
    if start_index < 0:
        raise ValueError('start_index must be positive')

    return source[start_index:start_index+symbol_count]


if __name__ == '__main__':
    original = 'GEEKSFORGEEKS'
    print(subject(original, 0, 5))
    print(subject(original, 5))