Difficulty with assigning values to bytearrays in Python without using integers

Question

I am trying to work with byte buffers in Python. The way I tried to go about it was to start with a zero-filled bytearray:

>>> d=bytearray(3)
>>> d
bytearray(b'\x00\x00\x00')

So far so good. I had tried to use bytes, but the documentation told me these would not be mutable. However, bytearrays should support mutable operations. Let's say I'm building some low-level fixed-sized packet, zero-padded, and I need to set the first bytes for my header. I try, because I'm lazy:

>>> d[0:1] = b'\x01\x01'
>>> d
bytearray(b'\x01\x01\x00\x00')

What gives? I started with a zero-filled bytearray precisely because I needed to keep the size constant. With the slicing I used, the answer doesn't even make sense: at worst, I expected the assignment of the two bytes to occur in both positions [0,1], adding two extra bytes to the total. Adding one extra byte just sounds nonsensical. Anyway...

Now, sure, let me try to assign each byte individually like a proper person...

>>> d[0] = b'\x01'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'bytes' object cannot be interpreted as an integer

I give up. I'm certainly not going to be working in both bytes and integers at the same time to fix this (and why was that not a problem in the previous example?). And I can't find any meaningful resource on the internet on "bytearray assignments" (the closest was a Wikiversity article which had an empty Assignments section).

Is there a sane way to manipulate bytearrays in Python? Thank you!

Python 3.7.6 (tags/v3.7.6:43364a7ae0, Dec 19 2019, 00:42:30) [MSC v.1916 64 bit (AMD64)] on win32

d[0:1] = b'\x01\x01' will always work that way with built-in slicing, silce notation is non inclusive. Bytearrays are mutable, and can re-size. You want d[0:2] = b'\x01\x01' Note, bytearray objects require int objects for assignment (regular and slice-based). Because slice-assigning with another bytes objects will produce ints, note list(b'\x01') — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Mar 12, 2020 at 11:39
Does this answer your question? How can i modify a byte array? — metatoaster
– metatoaster, Commented Mar 12, 2020 at 11:40
@juanpa.arrivillaga Ok, I'm glad that can work then, since it will make my work easier. Can I use d[0:1] for single byte assignments then? — Ricardo M.
– Ricardo M., Commented Mar 12, 2020 at 11:44

Ricardo M. · Accepted Answer · 2020-03-12 11:58:12Z

3

Thanks to @juanpa.arrivillaga's comment above.

You can work with bytes in bytearray if you use non-inclusive* slice assignments, such that:

d[0:1] = b'\x01' # Single byte assignment
d[0:2] = b'\x01\x01' # Multiple byte assignments

This type of byte assignment will implicitly produce the integers needed to assign to the bytearray, instead of e.g. d[0]. If the length of the slice is smaller than the assignment, the array will grow as in my example.

(*that's just the way to slice in Python, the emphasis is just because I forgot about it in the question.)

answered Mar 12, 2020 at 11:58

Ricardo M.

511 silver badge6 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Difficulty with assigning values to bytearrays in Python without using integers

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related