Python IndexError: index index out of range. Unable to access by index

Question

I use pySerialTTL for reading a stream of bytes. To read two bytes:

CheckSumByte = [ b for b in ser.read(2)]
print( CheckSumByte)
print( type(CheckSumByte))
print( str(len(CheckSumByte)))
print( CheckSumByte[0])

Conclusion:

[202, 87]
<class 'list'>
2
IndexError: list index out of range

I cannot access elements CheckSumByteby index (0 or 1). What's wrong?

Here is my code:

while(ReadBufferCount < 1000):
    time.sleep(0.00002)
    InputBuffer = ser.inWaiting()
    if (InputBuffer > 0):
        FirstByte = ser.read(1)
        if ord(FirstByte) == 0xFA:
            while ser.inWaiting() < 21: pass
        IndexByte = ser.read(1)
        SpeedByte = [ b for b in ser.read(2)]
        DataByte0 = [ b for b in ser.read(4)]
        DataByte1 = [ b for b in ser.read(4)]
        DataByte2 = [ b for b in ser.read(4)]
        DataByte3 = [ b for b in ser.read(4)]
        CheckSumByte = [ b for b in ser.read(2)]
        print( CheckSumByte[0]) #Out of Range??`

Traceback (most recent call last):

  File "<ipython-input-6-5233b0a578b1>", line 1, in <module>
    runfile('C:/Users/Blair/Documents/Python/Neato XV-11 Lidar/Serial9.py', wdir='C:/Users/Blair/Documents/Python/Neato XV-11 Lidar')

  File "C:\Program Files (x86)\WinPython-32bit-3.4.3.3\python-3.4.3\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 682, in runfile
    execfile(filename, namespace)

  File "C:\Program Files (x86)\WinPython-32bit-3.4.3.3\python-3.4.3\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 85, in execfile
    exec(compile(open(filename, 'rb').read(), filename, 'exec'), namespace)

  File "C:/Users/Blair/Documents/Python/Neato XV-11 Lidar/Serial9.py", line 88, in <module>
    print( CheckSumByte[0]) #Out of Range??

IndexError: list index out of range

Kenny: Thanks. Even simpler for two bytes:

    CheckSumByte.append(ser.read(1))
    CheckSumByte.append(ser.read(1))

It works correctly, but inconveniently. Elements are type bytes. How to add items to a list using list comprehension? I would like to avoid the append function because it is slow.

I notice that this does not work when the CheckSumByte elements are integer. In Python 3's understanding, does it need a special format for adding bytes to bytes (not for converting to an integer)?

Answer 1

, ser :

ser = serial.Serial(
    port=PortName, baudrate=115200, parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE, bytesize=serial.EIGHTBITS, 
    timeout=0)

, ser ( , !).

, , ser.read(n) n . , n , :

• ser ( timeout=None) ;
• ( )

, , , n , - :

def read_exactly(ser, n):
    bytes = b""

    while len(bytes) < n:
        bytes += ser.read(n - len(bytes))

    return bytes

, , , . , . , FirstByte != 0xFA , .

Answer 2

donkopotamus - . . timeout = 0, . , ser . = . . IndexError. .