When spn_start is a range cut_data is incorrectly retuned as None

[s5y3XZpGvQPApqR]

def get_spn_cut_bytes(spn_start, spn_length, message_data_bitstring, is_complete_message):
    spn_end = spn_start[0] + spn_length - 1
    if not is_complete_message and spn_end > message_data_bitstring.length:
        return bitstring.Bits(bytes=b'')

    cut_data = message_data_bitstring[spn_start[0]:spn_end + 1]
    if len(spn_start) > 1:
        lsplit = int(spn_start[1] / 8) * 8 - spn_start[0]
        rsplit = spn_length - lsplit
        cut_data = bitstring.BitArray(message_data_bitstring[spn_start[0]:spn_start[0] + lsplit]).append(
            message_data_bitstring[spn_start[1]:spn_start[1] + rsplit])
    return cut_data

if spn_start is a list e.g. [0, 8] requiring the first two bytes then the cut_data is incorrectly returned at None

Here's what I expect:
in log.txt
(1621774359.366587) can0 14FEBF80#1010010203040506

{
    "PGN": "EBC2(65215)",
    "DA": "All(255)",
    "SA": "Unknown(128)",
    "Front Axle Speed": "16.0625 [kph]",
    "Relative Speed; Front Axle, Left Wheel": "-7.75 [kph]",
    "Relative Speed; Front Axle, Right Wheel": "-7.6875 [kph]",
    "Relative Speed; Rear Axle #1, Left Wheel": "-7.625 [kph]",
    "Relative Speed; Rear Axle #1, Right Wheel": "-7.5625 [kph]",
    "Relative Speed; Rear Axle #2, Left Wheel": "-7.5 [kph]",
    "Relative Speed; Rear Axle #2, Right Wheel": "-7.4375 [kph]"
}

The first entry Front Axle Speed 16 kph is not returned in the current version. rsplit and lsplit are not correct and the cut_data assignment doesn't work even if they are 0 and 16.
This works for me but I'm not sure if this breaks other functionality

def get_spn_cut_bytes(spn_start, spn_length, message_data_bitstring, is_complete_message):
    print("cut bytes")
    spn_end = spn_start[0] + spn_length - 1
    if not is_complete_message and spn_end > message_data_bitstring.length:
        return bitstring.Bits(bytes=b'')

    cut_data = message_data_bitstring[spn_start[0]:spn_end + 1]
    if len(spn_start) > 1:
        lsplit = int(spn_start[0])
        rsplit = int(spn_start[1]) + 8
        cut_data = message_data_bitstring[lsplit:rsplit]
    return cut_data

[BenGardiner]

Thanks for bringing this up, @s5y3XZpGvQPApqR . Before the changes you propose the Front Axle Speed is missing from the description as you say

# python3 pretty_j1939.py --candata --real-time --format --da-json tmp/J1939DA_201910.json tmp/issue25.log
(1621774359.366587) can0 14FEBF80#1010010203040506 ; {
                                                   ;     "PGN": "EBC2(65215)",
                                                   ;     "DA": "All(255)",
                                                   ;     "SA": "Unknown(128)",
                                                   ;     "Relative Speed; Front Axle, Left Wheel": "-7.75 [km/h]",
                                                   ;     "Relative Speed; Front Axle, Right Wheel": "-7.688000000000001 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #1, Left Wheel": "-7.626 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #1, Right Wheel": "-7.564 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #2, Left Wheel": "-7.502000000000001 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #2, Right Wheel": "-7.44 [km/h]"
                                                   ; }

Then with your changes we get some front axle speed as described. Thank you for the testcase here!

# python3 pretty_j1939.py --candata --real-time --format --da-json tmp/J1939DA_201910.json tmp/issue25.log
cut bytes
cut bytes
cut bytes
cut bytes
cut bytes
cut bytes
cut bytes
(1621774359.366587) can0 14FEBF80#1010010203040506 ; {
                                                   ;     "PGN": "EBC2(65215)",
                                                   ;     "DA": "All(255)",
                                                   ;     "SA": "Unknown(128)",
                                                   ;     "Front Axle Speed": "16.0625 [km/h]",
                                                   ;     "Relative Speed; Front Axle, Left Wheel": "-7.75 [km/h]",
                                                   ;     "Relative Speed; Front Axle, Right Wheel": "-7.6875 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #1, Left Wheel": "-7.625 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #1, Right Wheel": "-7.5625 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #2, Left Wheel": "-7.5 [km/h]",
                                                   ;     "Relative Speed; Rear Axle #2, Right Wheel": "-7.4375 [km/h]"
                                                   ; }

The problem was introduced by me when I ported @j4l-Shvn's code in PR #6. Their code was fine. I didn't use bitstring's .append() the correct way. つ﹏⊂

I really appreciate you pointing out this problem, @s5y3XZpGvQPApqR . It also led me to create issues #28 and #27 during testing.