Problem with serial data. How do I ignor incorrect data?

[donmerch]

I'm reading data via USB serial @ 57600 baud. I'm trying to take 5 readings and then get the average. Occasionally I get an error and I don't know how to get around it.

Here's my code:

from serial import Serial

ser = Serial('/dev/ttyUSB0', 57600, 8, 'N', 1, timeout=1)

numPoints = 5
reading = [0] * numPoints
avgDistance = 0

def getValues():
    ser.flush()
    c = ser.read()
    print("c1:", c)
    if c == b'R':
        c = ser.read(5).decode('ascii').strip('\r')
        print("c2:", c)
        return int(c)
        
    ser.flush()

while 1:
    for i in range(0, numPoints):
        data = getValues()
        reading[i] = data
        avgDistance += data
    avgDistance = avgDistance / numPoints
    
    print(reading)
    print("Average:", avgDistance)
    break
    
ser.close()

And here's my output screen showing an error and a good read:

Python 3.4.2 (default, Oct 19 2014, 13:31:11) 
[GCC 4.9.1] on linux
Type "copyright", "credits" or "license()" for more information.
>>> ================================ RESTART ================================
>>> 
c1: b'\x00'
Traceback (most recent call last):
  File "/home/pi/Documents/Python Projects/PySerialCode/Lesson1E.py", line 24, in <module>
    avgDistance += data
TypeError: unsupported operand type(s) for +=: 'int' and 'NoneType'
>>> ================================ RESTART ================================
>>> 
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
[417, 417, 417, 417, 417]
Average: 417.0
>>>

Any constructive help much appreciated. Thanks.

Edit: The print c1 and print c2 are for debugging.

---

I should have mentioned- the message it's receiving looks like this: Rxxxx\r
The message begins with "R", has 4 ascii digits (xxxx) then ends with a "\r" line return.

I tried using ser.readline() but I couldn't get it to work. Does it only respond to "\n"? Or "\r\n"?

Thanks

[donmerch]

Okay here's what I've gotten to work thus far. I'd still like to know how I might ignore the "None" return and still get 5 full readings. Here's what I came up with :

from serial import Serial
import time

ser = Serial('/dev/ttyUSB0', 57600, 8, 'N', 1, timeout=3)

numPoints = 5
reading = [0] * numPoints
avgDistance = 0

def getValues():
    c = ser.read()
    print("c1:", c)         # used for debugging only
    if c != b'\x00':
        if c == b'R':
            c = ser.read(5).decode('ascii').rstrip('\r')
            print("c2:", c) # used for debugging only
            return int(c)
    else:
        print("Error")      # used for debugging only

while 1:
    for i in range(0, numPoints):
        data = getValues()
        if data is not None:
            reading[i] = data
            avgDistance += data
        else:
            numPoints = numPoints - 1   # reduce divisor by number of "None" returned
    avgDistance = avgDistance / numPoints
    
    print(reading)
    print("Average:", avgDistance)
    break
    
ser.close()

And here's some output:

>>> ================================ RESTART ================================
>>> 
c1: b'\x00'
Error
c1: b'\r'
c1: b'R'
c2: 0416
c1: b'R'
c2: 0416
c1: b'R'
c2: 0416
[0, 0, 416, 416, 416]
Average: 416.0
>>> ================================ RESTART ================================
>>> 
c1: b'\x00'
Error
c1: b'\r'
c1: b'R'
c2: 0416
c1: b'R'
c2: 0418
c1: b'R'
c2: 0417
[0, 0, 416, 418, 417]
Average: 417.0
>>> ================================ RESTART ================================
>>> 
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0417
c1: b'R'
c2: 0416
[417, 417, 417, 417, 416]
Average: 416.8
>>>