def get_quotechars(data, quote_chars=None):
    """Get potential quote characters

    Quote characters are those that occur in the ``quote_chars`` set and are 
    found at least once in the file.

    Parameters
    ----------

    data: str
        The data of the file as a string

    quote_chars: iterable
        Characters that should be considered quote characters. If it is None, 
        the following default set is used::

        ["'", '"', "~", "`"]


    Returns
    -------
    quotes: set
        Set of potential quote characters. The empty string is added by 
        default.

    """
    if quote_chars is None:
        quote_chars = ["'", '"', "~", "`"]
    Q = set(quote_chars) & set(data)
    Q.add("")
    return Q