How to use the dulwich.errors.GitProtocolError function in dulwich

def read_pkt_refs(proto):
    server_capabilities = None
    refs = {}
    # Receive refs from server
    for pkt in proto.read_pkt_seq():
        (sha, ref) = pkt.rstrip("\n").split(None, 1)
        if sha == "ERR":
            raise GitProtocolError(ref)
        if server_capabilities is None:
            (ref, server_capabilities) = extract_capabilities(ref)
            symref = "symref=HEAD:"
            for cap in server_capabilities:
                if cap.startswith(symref):
                    sha = cap.replace(symref, "")
        refs[ref] = sha

    if len(refs) == 0:
        return None, set([])
    return refs, set(server_capabilities)

def read_pkt_refs(proto):
    server_capabilities = None
    refs = {}
    # Receive refs from server
    for pkt in proto.read_pkt_seq():
        (sha, ref) = pkt.rstrip(b'\n').split(None, 1)
        if sha == b'ERR':
            raise GitProtocolError(ref)
        if server_capabilities is None:
            (ref, server_capabilities) = extract_capabilities(ref)
        refs[ref] = sha

    if len(refs) == 0:
        return None, set([])
    return refs, set(server_capabilities)

allowed return values.
    """
    if not line:
        fields = [None]
    else:
        fields = line.rstrip(b'\n').split(b' ', 1)
    command = fields[0]
    if allowed is not None and command not in allowed:
        raise UnexpectedCommandError(command)
    if len(fields) == 1 and command in (COMMAND_DONE, None):
        return (command, None)
    elif len(fields) == 2:
        if command in (COMMAND_WANT, COMMAND_HAVE, COMMAND_SHALLOW,
                       COMMAND_UNSHALLOW):
            if not valid_hexsha(fields[1]):
                raise GitProtocolError("Invalid sha")
            return tuple(fields)
        elif command == COMMAND_DEEPEN:
            return command, int(fields[1])
    raise GitProtocolError('Received invalid line from client: %r' % line)

def write_pkt_line(self, line):
        """Sends a pkt-line to the remote git process.

        Args:
          line: A string containing the data to send, without the length
            prefix.
        """
        try:
            line = pkt_line(line)
            self.write(line)
            if self.report_activity:
                self.report_activity(len(line), 'write')
        except socket.error as e:
            raise GitProtocolError(e)

self._readahead = None

        try:
            sizestr = read(4)
            if not sizestr:
                raise HangupException()
            size = int(sizestr, 16)
            if size == 0:
                if self.report_activity:
                    self.report_activity(4, 'read')
                return None
            if self.report_activity:
                self.report_activity(size, 'read')
            pkt_contents = read(size-4)
        except socket.error as e:
            raise GitProtocolError(e)
        else:
            if len(pkt_contents) + 4 != size:
                raise GitProtocolError(
                    'Length of pkt read %04x does not match length prefix %04x'
                    % (len(pkt_contents) + 4, size))
            return pkt_contents

self._readahead = None

        try:
            sizestr = read(4)
            if not sizestr:
                raise HangupException()
            size = int(sizestr, 16)
            if size == 0:
                if self.report_activity:
                    self.report_activity(4, 'read')
                return None
            if self.report_activity:
                self.report_activity(size, 'read')
            pkt_contents = read(size-4)
        except socket.error as e:
            raise GitProtocolError(e)
        else:
            if len(pkt_contents) + 4 != size:
                raise GitProtocolError(
                    'Length of pkt read %04x does not match length prefix %04x'
                    % (len(pkt_contents) + 4, size))
            return pkt_contents

def write_pkt_line(self, line):
        """Sends a pkt-line to the remote git process.

        :param line: A string containing the data to send, without the length
            prefix.
        """
        try:
            line = pkt_line(line)
            self.write(line)
            if self.report_activity:
                self.report_activity(len(line), 'write')
        except socket.error as e:
            raise GitProtocolError(e)

def __init__(self, *args, **kwargs):
        Exception.__init__(self, *args, **kwargs)


class GitProtocolError(Exception):
    """Git protocol exception."""

    def __init__(self, *args, **kwargs):
        Exception.__init__(self, *args, **kwargs)


class SendPackError(GitProtocolError):
    """An error occurred during send_pack."""


class UpdateRefsError(GitProtocolError):
    """The server reported errors updating refs."""

    def __init__(self, *args, **kwargs):
        self.ref_status = kwargs.pop('ref_status')
        super(UpdateRefsError, self).__init__(*args, **kwargs)


class HangupException(GitProtocolError):
    """Hangup exception."""

    def __init__(self):
        super(HangupException, self).__init__(
            "The remote server unexpectedly closed the connection.")


class UnexpectedCommandError(GitProtocolError):

sizestr = read(4)
            if not sizestr:
                raise HangupException()
            size = int(sizestr, 16)
            if size == 0:
                if self.report_activity:
                    self.report_activity(4, 'read')
                return None
            if self.report_activity:
                self.report_activity(size, 'read')
            pkt_contents = read(size-4)
        except socket.error as e:
            raise GitProtocolError(e)
        else:
            if len(pkt_contents) + 4 != size:
                raise GitProtocolError(
                    'Length of pkt read %04x does not match length prefix %04x'
                    % (len(pkt_contents) + 4, size))
            return pkt_contents