import copy


def _str_params(params):
    """
    Convert NoneType values to the string 'None'
    for display.

    Parameters
    ----------
    params: dict
        A dictionary of input values to a function
    """
    params_str = copy.deepcopy(params)
    for k, v in params_str.items():
        if v is None:
            params_str[k] = 'None'
    return params_str