def getStringParams(param_str):
    """Determines which keys are required if param_str.format() function is used.
       Returns the required keys as a list.
    """
    params = []
    index = 0
    while True:
        index = param_str.find("{", index)
        if index >= 0:
            end_index = param_str.find("}", index)
            if end_index > index:
                double_colon_index = param_str.find(":", index)
                if index < double_colon_index < end_index:
                    params.append(param_str[index+1:double_colon_index])
            index += 1
        else:
            break

    return params