During my work, i have to use a python client to talk to a protobuf server based on netty. I can not find some python helpers to parse the length prepender that netty adds to each protobuf message. So I wrote the following code.

python_client_for_netty_protobuf.py

# the following is talk to protobuf server based on netty 4.1.11.Final

ONE_BYTE_MASK = 0xffffffff << 7
TWO_BYTES_MASK = 0xffffffff << 14
THREE_BYTES_MASK = 0xffffffff << 21
FOUR_BYTES_MASK = 0xffffffff << 28

def how_many_bytes_this_length_take(package_len):
    result = package_len & ONE_BYTE_MASK
    if result == 0:
        return 1

    result = package_len & TWO_BYTES_MASK
    if result == 0:
        return 2

    result = package_len & THREE_BYTES_MASK
    if result == 0:
        return 3

    result = package_len & FOUR_BYTES_MASK
    if result == 0:
        return 4

    return 5

def get_packet_len(sock):
    one_byte = sock.recv(1)
    one_byte_value, = unpack(">B", one_byte)
    if one_byte_value >= 0:
        return one_byte_value

    result = one_byte_value & 127
    one_byte = sock.recv(1)
    one_byte_value, = unpack(">B", one_byte)
    if one_byte_value >= 0:
        one_byte_value = one_byte_value << 7
        return result | one_byte_value

    one_byte_value = (one_byte_value & 127) << 7
    result = result | one_byte_value

    one_byte = sock.recv(1)
    one_byte_value, = unpack(">B", one_byte)
    if one_byte_value >= 0:
        one_byte_value = one_byte_value << 14
        return result | one_byte_value

    one_byte_value = (one_byte_value & 127) << 14
    result = result | one_byte_value

    one_byte = sock.recv(1)
    one_byte_value, = unpack(">B", one_byte)
    if one_byte_value >= 0:
        one_byte_value = one_byte_value << 21
        return result | one_byte_value

    one_byte_value = (one_byte_value & 127) << 21
    result = result | one_byte_value

    one_byte = sock.recv(1)
    one_byte_value, = unpack(">B", one_byte)
    if one_byte_value < 0:
        raise Exception("malformed package")

    one_byte_value = (one_byte_value & 127) << 28
    return  result | one_byte_value


def generate_length_prepender(length):
    num_of_bytes = how_many_bytes_this_length_take(length)
    format_str = ">"
    for i in range(num_of_bytes):
        format_str += "B"

    return pack(format_str, length)