Fixing escaped characters python-xbee

[MRAB]

On 24/04/2013 08:33, pabloblo85 at gmail.com wrote:
> I am using a XBee to receive data from an arduino network.
>
> But they have AP=2 which means escaped characters are used when a 11 or 13 appears (and some more...)
>
> When this occurs, XBee sends 7D and inmediatly XOR operation with char and 0x20.
>
> I am trying to recover the original character in python but I don't know ho to do it.
>
> I tried something like this:
>
> 	read = ser.read(4) #Read 4 chars from serial port
> 	for x in range (0,4):
> 		if(toHex(read[x]) != '7d'): #toHex converts it to hexadecimal just for checking purposes
> 			if(x < 3):
> 				read[x] = logical_xor(read[x+1], 20) #XOR
> 				for y in range (x+1,3):
> 					read[y] = read[y+1]					
> 				read[3] = ser.read()
> 			else:
> 				read[x] = logical_xor(ser.read(), 20) #XOR
> 	
> 	data = struct.unpack('<f', read)[0]
>
> logical_xor is:
>
> def logical_xor(str1, str2):
>      return bool(str1) ^ bool(str2)
>
> I check if 7D character is in the first 3 chars read, I use the next char to convert it, if it is the 4th, I read another one.
>
> But I read in python strings are inmutables and I can't change their value once they have one.
>
> What would you do in this case? I started some days ago with python and I don't know how to solve this kind of things...
>
You could try converting to a list, which is mutable.

# Python 3
read = list(ser.read(4))

pos = 0
try:
     while True:
         pos = read.index(0x7D, pos)
         del read[pos]
         read.extend(ser.read())
         read[pos] ^= 0x20
except ValueError:
     # There are no (more) 0x7D in the data.
     pass

data = struct.unpack('<f', bytes(read))[0]


# Python 2
read = [ord(c) for c in ser.read(4)]

pos = 0
try:
     while True:
         pos = read.index(0x7D, pos)
         del read[pos]
         read.append(ord(ser.read()))
         read[pos] ^= 0x20
except ValueError:
     # There are no (more) 0x7D in the data.
     pass

data = struct.unpack('<f', b"".join(chr(c) for c in read))[0]