[Tutor] Help with building bytearray arrays

[Cameron Simpson]

On 07Sep2018 15:45, Chip Wachob <wachobc at gmail.com> wrote:
>Basically I'm trying to write a block of unsigned bytes to the device
>and read back an equal sized block of unsigned bytes.  There's a
>function that is provided called transfer(data_to_send, num_of_bytes)
>that handles the heavy lifting.  Unfortunately there seems to be a bug
>in the part and if I attempt to send the entire block of bytes (64),
>the device will lock up.  I've been able to determine that if I send
>16 bytes at a time, I'm okay.
>
>So, I take my bytearray(64) and step through it 16 bytes at a time like this:
>
>my function's main pieces are:
>
>def transfer_byte_array():
>   MAX_LOOP_COUNT = 64
>   slice_size = 16
>   read_ary = bytearray(MAX_LOOP_COUNT)
>   scratch_ary = bytearray()
>
>   for step in range (0, MAX_LOOP_COUNT, slice_size):
>      scratch_ary = transfer(data_to_send, slice_size)
>
>      for bytes in range (0, slice_size):
>         read_ary = scratch_ary[bytes]
>
>   return(read_ary)
>
>
>Ideally, I'd like to take the slice_size chunks that have been read
>and concatenate them back togetjer into a long MAX_LOOP_COUNT size
>array to pass back to the rest of my code.  Eg:
>
>read_ary = ary_slice[0] + ary_slice[1] + ary_slice[2] + ary_slice[3]

Minor remark: don't use the name "bytes" for a variable, it is a builtin type 
name and you're shadowing it.

It looks to me like "transfer" hands you back a buffer with the read data, so 
this:

  scratch_ary = bytearray()

don't do anything (it gets discarded).

If you're getting back a bytes or bytearray object from transfer, just gather 
them all up in an list:

  returned_buffers = []
  for ......
      response = transfer(data_to_send, slice_size)
      returned_buffers.append(response)
  .......
  read_ary = b''.join(returned_buffers)

Note that that makes a new bytes object for read_ary to refer to. You don't 
need the earlier initialisation of read_ary.

Also note that the bytes object is read only; if that is a problem you'll need 
to construct a bytearray instead.

[...]
>The problem that I repeatedly run into is with the line:
>
>read_ary = scratch_ary[bytes]  (or variants thereof)
>
>The traceback is this:
>
>Traceback (most recent call last):
>  File "SW8T_5.py", line 101, in <module>
>    loop_size = RSI_size_the_loop(Print)
>  File "/home/temp/Python_Scratch/examples/RSI.py", line 350, in
>RSI_size_the_loop
>    read_ary.append(scratch_ary[singles])
>TypeError: an integer or string of size 1 is required

Yeah I thought that looked weird to me too. 

>or, one of the other common ones that I've seen is
>
>TypeError: can't concat bytearray to list
>
>This one is confusing because both of the operands are bytearry
>types.. or at least I thought they should be...

No, one will be a list :-) putting a bunch of:

  print(repr(foo))

replacing "foo" with relevant variables will be illuminating to you; you can 
see immediately where this are not what you expected.

>I'm obviously missing something fundamental here.  Problem is I can't
>seem to find any examples of people asking this question before on the
>inter-webs..

You have the opposite of my problem. I can often find people asking the same 
question, but less often an answer. Or a decent answer, anyway.

Cheers,
Cameron Simpson <cs at cskk.id.au>