[Tutor] Help with building bytearray arrays

[Cameron Simpson]

Please try to adopt the inline reply style; we prefer it here. It lets us reply 
point by point and makes messages read like conversations. Anyway...

On 07Sep2018 23:57, Chip Wachob <wachobc at gmail.com> wrote:
>Point taken on 'bytes'..  thanks.
>
>the
>scratch_ary = bytearray()
>
>was my attempt at 'setting' the type of the variable.  I had hoped
>that it would help resolve the error messages telling me that the
>types didn't go together.
>Coming from a 'C' background, I find the lack of typing in Python to
>be confusing.  I'm used to working with bytes / words signed and
>unsigned for a reason.

Ok. Variables aren't typed. Variables are references to objects, which _are_ 
typed. So pointing a variable at a bytearray doesn't set its type in any 
persistent sense.

Thus:

  x = 1
  x = "a string"
  x = bytearray()

All just point "x" at different objects. If you'd like a C metaphor, variables 
are _like_ (void*) pointers: they can reference any kind of object.

Aside: they're not pointers, avoid the term - people will complain. As far as 
the language spec goes they're references. Which may in a particular 
implementation be _implemented internally_ using pointers.

>So, if I'm understanding the transfer() function correctly, the
>function takes and returns a bytearray type.

It would be good to see the specification for the transfer function. They we 
can adhere to its requirements. Can you supply a URL?

>You mentioned about
>constructing a bytearray if I need one.  Can you expand on how I
>approach that?

Well, you were getting several bytes or bytearray objects back from transfer 
and wanted to join them together. The efficient way is to accumulate them in a 
list and then join them all together later using the bytearry (or bytes) .join 
method:

  https://docs.python.org/3/library/stdtypes.html#bytearray.join

So:

  data_list = []
  for ....
    # send some data, get some data back
    data = transfer(...)
    # add that buffer to the data_list
    data_list.append(data)
  # make a single bytes object being the concatenation of all the smaller data 
  # chunks
  all_data = bytes.join(data_list)

It looks to me like your transfer() function is handed a bytes or bytearray and 
returns one. Normally that would be a separate bytes or bytearray.

Aside: bytes and bytearray objects are generally the way Python 3 deals with 
chunks of bytes. A "bytes" is readonly and a bytearray may have its contents 
modified. From a C background, they're like an array of unsigned chars.

>I'm going to try the experiment you mentioned in hopes of it giving me
>a better understanding of the 'types' and what happens with the
>variables.

Sounds good.

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