How could C transfer to Python

Python
1

Python Language:
…
mbrtu = “”
mbrtu += chr(1)
mbrtu += chr(3)
mbrtu += chr(1)
mbrtu += chr(0)
mbrtu += chr(0)
mbrtu += chr(2)
crc = calcString( mbrtu, INITIAL_MODBUS)
print hex(crc) #output 0xf7c5
mbrtu += chr(crc & 0xFF) + chr((crc >> 8) & 0xFF)

sd.sendto(mbrtu, 0, DESTINATION)
payload, src_addr = sd.recvfrom(100)

#error here
Data = (int(payload[3]<<8)) + (int(payload[4]))
#TypeError: unsupported operand type(s) for <<: ‘str’ and ‘int’

#pValue = (float(Data))/power(10,1)
#print “received:”, pValue

C Language: it works
…
cmdBuf[0] = 1 ;
cmdBuf[1] = 3 ;
cmdBuf[2] = 1 ;
cmdBuf[3] = 0 ;
cmdBuf[4] = 0 ;
cmdBuf[5] = 2 ;
CRC = CRC16_T(cmdBuf,Len);
cmdBuf[6] = 0xFF & CRC ;
cmdBuf[7] = 0xFF & (CRC>>8) ;
write(fd,cmdBuf,Len+2);
ret=read(fd,rbuf,100);
Data = ((int)rbuf[3]<<8) + ((int) rbuf[4]);

How could I transfer C to Python language?
Sorry,it is my first time use Python language.

2

I don’t know much python, but I think the << operator takes precedence over casting (if python supports casting?) so maybe it should read:

Data = ((int(payload[3]))<<8) + (int(payload[4]))

3

mcarver’s answer looks almost correct. payload[3]<<8 will fail because you cannot shift Python characters - unlike in C where a char == byte == int, in Python it is a string of length=1.

However, I think even int(‘A’) will fail because ‘A’ isn’t a digit. Doing "Data = int(‘1’)’ sets Data to 1, not the ascii-bit equivalent of 0x31.

The normal Python way is this:
Data = (ord(payload[3])<<8) + ord(payload[4])

Or some people use the Python ‘struct’ module, but that is overkill for a simple digit.