Python 3：如何将附加到字节数组的数字附加为字节

Question

What I'd want to do in a Python script is have a bytearray and append two numbers to it, send it as a message, and have the receiving C application can read the number again.

The C app reads like this:

//deserialize srvid from end of payload
UInt16 srvIdFrom;
UInt16 srvIdTo;

srvIdFrom = payload[len-4]  | payload[len-3] << 8;
srvIdTo = payload[len-2]  | payload[len-1] << 8;

And I made a Python script that tries to do the above like this:

my_bytes = bytearray()
numb = 1
dummySrvId = 1234
srvIdFrom = 5678
my_bytes.append(numb)
my_bytes.append(dummySrvId & 0xff)
my_bytes.append(dummySrvId >> 8)
my_bytes.append(srvIdFrom & 0xff)
my_bytes.append(srvIdFrom >> 8)

but it does not work. Ie, the following code gives the following output:

srvIdFrom = my_bytes[len(my_bytes)-4] << 8 | my_bytes[len(my_bytes)-3] << 0
srvIdTo = my_bytes[len(my_bytes)-2] << 8 | my_bytes[len(my_bytes)-1] << 0
print('return packet had srvIdFrom {} and srvIdTo {}'.format(srvIdFrom,srvIdTo))

Output

return packet had srvIdFrom 53764 and srvIdTo 44

Or stuff similar to that. What am I doing wrong?

Answer 1

You appended the bytes in the following order

my_bytes.append(dummySrvId & 0xff)  # -4 (offset from end)
my_bytes.append(dummySrvId >> 8)    # -3
my_bytes.append(srvIdFrom & 0xff)   # -2
my_bytes.append(srvIdFrom >> 8)     # -1

but in your Python test to decode, you then shift the wrong bytes , effectively swapping the MSB and LSB parts:

srvIdFrom = (
    my_bytes[len(my_bytes)-4] << 8   #  -4 is dummySrvId & 0xff
  | my_bytes[len(my_bytes)-3] << 0   #  -3 is srvIdFrom >> 8
)
srvIdTo = (
    my_bytes[len(my_bytes)-2] << 8   #  -2 is srvIdFrom & 0xff
  | my_bytes[len(my_bytes)-1] << 0   #  -1 is srvIdFrom >> 8
)

The latter does not match your C code either; you shift the little-endian bytes (offsets -3 and -1):

srvIdFrom = payload[len-4]  | payload[len-3] << 8;
srvIdTo = payload[len-2]  | payload[len-1] << 8;

and indeed, if you swap the shifting and match your C code to decode, you get the correct values:

>>> my_bytes[len(my_bytes)-4] | my_bytes[len(my_bytes)-3] << 8
1234
>>> my_bytes[len(my_bytes)-2] | my_bytes[len(my_bytes)-1] << 8
5678

so something else is wrong . Your Python encoding work is fine.