Convert 0 to 255 to single Char

Question

I am trying to convert values from 0 to 255 to a single char. I am using chr. But this is somehow converting a number like 255 to a two char string. I add up those values to a string which is then converted with bytes(string , 'UTF-8') into a bytestream and written over serial to UART.

#!/usr/bin/python
import os
import sys
import serial
def asbyte(v):
 return chr(v & 0xFF)
def printStdErr(*objs):
# print("", *objs, file=stderr)
 print("")
class LightYModem:
 soh = 1 # 128 byte blocks
 stx = 2 # 1K blocks
 eot = 4
 ack = 6
 nak = 0x15
 ca = 0x18 # 24
 crc16 = 0x43 # 67
 abort1 = 0x41 # 65
 abort2 = 0x61 # 97
 packet_len = 1024
 expected_packet_len = packet_len+5
 packet_mark = stx
 def __init__(self):
 self.seq = None
 self.ymodem = None
 def blocking_read(self):
 ch = ''
 while not ch:
 ch = self.ymodem.read(1)
 printStdErr("read %d " % ord(ch))
 return ch
 def write(self, packet):
 #print(packet)
 b = bytes(packet, 'UTF-8')
 print(b)
 self.ymodem.write(b)
 return len(packet);
 def flush(self):
 pass
 #self.ymodem.flush()
 def _read_response(self):
 ch1 = self.blocking_read()
 ch1 = ord(ch1)
 printStdErr("response %d" % (ch1))
 if ch1==LightYModem.ack and self.seq==0: # may send also a crc16
 ch2 = self.blocking_read()
 elif ch1==LightYModem.ca: # cancel, always sent in pairs
 ch2 = self.blocking_read()
 return ch1
 def transfer(self, file, ymodem, output):
 self.ymodem = ymodem
 """
 file: the file to transfer via ymodem
 ymodem: the ymodem endpoint (a file-like object supporting write)
 output: a stream for output messages
 """
 file.seek(0, os.SEEK_END)
 size = file.tell()
 file.seek(0, os.SEEK_SET)
 response = self.send_filename_header("binary", size)
 while response==LightYModem.ack:
 response = self.send_packet(file, output)
 file.close()
 if response==LightYModem.eot:
 self._send_close()
 return response
 def _send_ymodem_packet(self, data):
 # pad string to 1024 chars
 data = data.ljust(LightYModem.packet_len)
 seqchr = asbyte(self.seq & 0xFF)
 print("seqchr : "+str(len(seqchr)))
 seqchr_neg = asbyte((255-self.seq) & 0xFF)
 print("seqchr_neg : "+str(len(seqchr_neg)))
 crc16 = '\x00\x00'
 print("bytes "+ str(bytes([255])))
 packet = asbyte(LightYModem.packet_mark) + seqchr + seqchr_neg + data + crc16
 if len(packet)!=LightYModem.expected_packet_len:
 raise Exception("packet length is wrong!")
 self.packet = packet
 written = self.write(packet)
 printStdErr("sent packet data, flush..."+str(written))
 self.flush()
 printStdErr("wait response..")
 response = self._read_response()
 if response==LightYModem.ack:
 ("sent packet nr %d " % (self.seq))
 self.seq += 1
 return response
 def send_packet(self, file, output):
 response = LightYModem.eot
 data = file.read(LightYModem.packet_len)
 if len(data):
 response = self._send_ymodem_packet(data)
 return response
 
 def send_filename_header(self, name, size):
 self.seq = 0
 packet = name 
 packet += asbyte(0) 
 packet += str(size) 
 packet += ' '
 return self._send_ymodem_packet(packet)
 def transfer(self, file, ymodem, output):
 self.ymodem = ymodem
 """
 file: the file to transfer via ymodem
 ymodem: the ymodem endpoint (a file-like object supporting write)
 output: a stream for output messages
 """
 file.seek(0, os.SEEK_END)
 size = file.tell()
 file.seek(0, os.SEEK_SET)
 response = self.send_filename_header("binary", size)
 while response==LightYModem.ack:
 response = self.send_packet(file, output)
 file.close()
 if response==LightYModem.eot:
 self._send_close()
 return response
def ymodem(args):
 ser = serial.Serial('COM24', 115200, serial.EIGHTBITS, serial.PARITY_NONE, serial.STOPBITS_ONE)
 os.chdir(r'D:\projects\testprojects\code\Test1\config')
 file = open(os.getcwd()+'\testfile.bin', 'rb')
 result = LightYModem().transfer(file, ser, sys.stderr)
 file.close()
 print("result: " + str(result))
 try:
 while (True):
 print(ser.read())
 except:
 pass
 print("Done")
if __name__ == '__main__':
 ymodem(sys.argv)

Tryed to build the bytes - Array for the different elements and added the 0-255 as uint8. Then I cannot see anything on the STM32 I use as a recepient over UART.

UPDATE: When I use bytes with LATIN-1 I get the 0 to 255 in a single byte transferred. If i try to send a single element from a LATIN-1 encoded bytearray, it doesnt work.Why is that so?

 seqchr ='\x00'
 seqchr_neg = '\xFF'
 packet = seqchr + seqchr_neg 
...
 def write(self, packet):
 bh = bytearray(packet, 'LATIN-1')
 ch = bytes(packet[0], 'LATIN-1')
 print(ch)
 print(bh[0])
 self.ymodem.write(bh )#receive only zero on my STM
 self.ymodem.write(ch)# receive the 0x02 on my STM

• chepner: chr returns a str value. That value can be encoded to a one- or two-byte bytes value depending on what encoding is used.

Answer 1

You don't want to use chr to get a string that might have a two-byte UTF-8 encoding; you just want the literal bytes value for a single 8-bit integer.

>>> bytes([230])
b'\xe6'

or

>>> import struct
>>> struct.pack('B', 230)
b'\xe6'

• user3719581: bytes endoded in LATIN-1 works for me, when sending it and receiving it via uart with my stm32. bytearray encoded in LATIN-1 does not. Why is that so?
• chepner: I'd have to see exactly what you are doing. For the most part, the only difference between bytes and bytearray is mutability. Both represent integer values between 0 and 255, inclusive.
• user3719581: I updated my question with the code excerpt