def _inet6_ntop(addr):
    """Convert an IPv6 address from binary form into text representation,
used when socket.inet_pton is not available.

    """
    # IPv6 addresses have 128bits (16 bytes)
    if len(addr) != 16:
        raise ValueError("invalid length of packed IP address string")

    # Decode to hex representation
    address = ":".join(plain_str(bytes_hex(addr[idx:idx + 2])).lstrip('0') or '0'  # noqa: E501
                       for idx in range(0, 16, 2))

    try:
        # Get the longest set of zero blocks. We need to take a look
        # at group 1 regarding the length, as 0:0:1:0:0:2:3:4 would
        # have two matches: 0:0: and :0:0: where the latter is longer,
        # though the first one should be taken. Group 1 is in both
        # cases 0:0.
        match = max(_IP6_ZEROS.finditer(address),
                    key=lambda m: m.end(1) - m.start(1))
        return '{}::{}'.format(address[:match.start()], address[match.end():])
    except ValueError:
        return address