I'm debugging a code written by another developer. I have a bitstream of data of different sizes which is written to a binary file as follows:

    bitstream.add(0x01, 8)
    bitstream.add(0x01, 32) # write data unit dummy size

    par_id, ps_id = 16
    bitstream.add(par_id, 12)
    bitstream.add(ps_id, 12)

where the `add` function is the following:

    def add(self, data, length):
        s = ''
        if (type(data) == str):
            for char in data:
                b = bin(ord(char))[2:]
                s = s + "{:0>8}".format(b)
        else:
            s = bin(data)[2:]
            if (len(s) < length):
                resto = length - len(s)
                for _ in range(0, resto):
                    s = '0' + s

        s = s[0:length]

        self.cache = self.cache + s
        self.flush()

The bitstream then contains

 - 8 bits -> value 0x01
 - 32 bits -> value 0x01
 - 12 bits -> value 16
 - 12 bits -> value 16

I have some issues in decoding the bitstream

    # unpack first 1 byte/8 bits
    data1, = struct.unpack('>B', in_file.read(1)) # value 1

    # unpack 2 bytes/16 bits
    number_tmp = struct.unpack('>H', in_file.read(2))[0] # value 16

    # set the 16th bit in number_tmp
    data2 = (data1 << 16) | number_tmp # value 65552

    # get last 5 bits in data2
    ps.parent_id = data2 >> 12 # value 16

    # bit mask 12 bits
    ps.id = data2 & 0x000FFF # value 16

What it is not clear to me:

 1. **data1** contains the first 8 bits and its value is 1 **OK**
 2. **number_tmp** contains the following 16 bits, but its value is not 1 **NOT OK**

**OBS**: I'm expecting to read the next 32 bits of the bitstream flow described above, instead it appears these 32 bits are ignored, since `number_tmp` evaluates to 16.


 3. **data2** is needed to set the 16th bit of `number_tmp`. I'm totally new to this kind of operations, so it is not really clear what is the purpose.
 4. **ps.parent_id** is set to the decimal value of the last 5 bits of `data2`
 5. **ps.id** is set to the decimal value of the first 5 bits of `data2`

Could anyone put light in my issues? I've read some tutorial but I could not find a proper explanation of what happens in my code.