Calculating CRC16 in Python

crcmod is working fine. You are not giving it the three bytes you think you are giving it. Your str(int(0x5A0001)) is providing seven bytes, which are the ASCII characters 5898241 — the conversion of 0x5a0001 to decimal.

To feed it the bytes 0x5a 0x00 0x01, you would instead (as one approach):

print hex(crc16("5a0001".decode("hex")))

That prints 0xace.

Here is a python implementation of CRC-16/CCITT-FALSE

def crc16(data : bytearray, offset , length):
    if data is None or offset < 0 or offset > len(data)- 1 and offset+length > len(data):
        return 0
    crc = 0xFFFF
    for i in range(0, length):
        crc ^= data[offset + i] << 8
        for j in range(0,8):
            if (crc & 0x8000) > 0:
                crc =(crc << 1) ^ 0x1021
            else:
                crc = crc << 1
    return crc & 0xFFFF

• data : bytearray of the data you want to calculate CRC for
• offset : from which offset you want to start calculating CRC
• length : to which offset you want to calculate CRC

A working single function example for CRC-16-ANSI, CRC-16-IBM based on [pycrc](<https://pypi.org/project/pycrc/) code.

It is easy to modify but input or output reflection capability is not included:

def crc16(data: bytes):
    xor_in = 0x0000  # initial value
    xor_out = 0x0000  # final XOR value
    poly = 0x8005  # generator polinom (normal form)

    reg = xor_in
    for octet in data:
        # reflect in
        for i in range(8):
            topbit = reg & 0x8000
            if octet & (0x80 >> i):
                topbit ^= 0x8000
            reg <<= 1
            if topbit:
                reg ^= poly
        reg &= 0xFFFF
        # reflect out
    return reg ^ xor_out

def crc16(data : bytearray, offset, length):
    if data is None or offset < 0 or offset > len(data) - 1 and offset + length > len(data):
        return 0
    print("uzunluk=", len(data))
    print(data)

    crc = 0xFFFF
    for i in range(length):
        crc ^= data[offset + i]
        for j in range(8):
            print(crc)
            if ((crc & 0x1) == 1):
                print("bb1=", crc)
                crc = int((crc / 2)) ^ 40961
                print("bb2=", crc)
            else:
                crc = int(crc / 2)
    return crc & 0xFFFF