How to use the fastecdsa.encoding.der.DEREncoder.decode_signature function in fastecdsa

def convert_der_sig(signature, as_hex=True):
    """
    Convert DER encoded signature to signature string

    :param signature: DER signature
    :type signature: bytes
    :param as_hex: Output as hexstring
    :type as_hex: bool

    :return bytes, str: Signature
    """

    if not signature:
        return ""
    if USE_FASTECDSA:
        r, s = DEREncoder.decode_signature(bytes(signature))
    else:
        sg, junk = ecdsa.der.remove_sequence(signature)
        if junk != b'':
            raise EncodingError("Junk found in encoding sequence %s" % junk)
        r, sg = ecdsa.der.remove_integer(sg)
        s, sg = ecdsa.der.remove_integer(sg)
    sig = '%064x%064x' % (r, s)
    if as_hex:
        return sig
    else:
        return binascii.unhexlify(sig)