In a recent python-ideas discussion of the differences between concatenation and augmented assignment on lists, I pointed out the general guiding principle behind Python's binary operation semantics was that the type of a binary operation should not depend on the order of the operands. That is "X op Y" and "Y op X" should either consistently create results of the same type ("1 + 1.1", "1.1 + 1") or else throw an exception ("[] + ()", "() + []").

This principle is why list concatenation normally only works with other lists, but will accept arbitrary iterables for augmented assignment. collections.deque exhibits similar behaviour (i.e. strict on the binary operation, permissive on augmented assignment).

However, bytes and bytearray don't follow this principle - they accept anything that implements the buffer interface even in the binary operation, leading to the following asymmetries:

>>> b'' + bytearray()
b''
>>> b'' + memoryview(b'')
b''
>>> bytearray() + b''
bytearray(b'')
>>> bytearray() + memoryview(b'')
bytearray(b'')
>>> memoryview(b'') + b''
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'memoryview' and 'bytes'
>>> memoryview(b'') + bytearray(b'')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'memoryview' and 'bytearray'

Now, the latter two cases are due to a known problem where returning NotImplemented from sq_concat or sq_repeat doesn't work properly (so none of the relevant method implementations in the stdlib even try), but the bytes and bytearray interaction is exactly the kind of type asymmetry the operand order independence guideline is intended to prevent.

My question is - do we care enough to try to change this? If we do, then it's necessary to decide on more appropriate semantics:

1. The "list" solution, permitting only the same type in binary operations (high risk of breaking quite a lot of code)
2. Don't allow arbitrary buffers, but do allow bytes/bytearray interoperability
  2a. always return bytes from mixed operations
  2b. always return bytearray from mixed operations
  2c. return the type of the first operand (ala set.__or__)

Or just accept that this really is more of a guideline than a rule and adjust the documentation accordingly.

I think the current behaviour is fine, in that the alternatives are not better at all. In the absence of a type inherently "superior" to the others (as float can be to int, except for very large integers :-)), it makes sense to keep the type of the left-hand argument.

Note that .join() has a slightly different behaviour:

>>> b"".join([bytearray(), b""])
b''
>>> bytearray().join([bytearray(), b""])
bytearray(b'')
>>> b"".join([bytearray(), memoryview(b"")])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sequence item 1: expected bytes, memoryview found

> Note that .join() has a slightly different behaviour:
> 
> >>> b"".join([bytearray(), b""])
> b''
> >>> bytearray().join([bytearray(), b""])
> bytearray(b'')
> >>> b"".join([bytearray(), memoryview(b"")])
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: sequence item 1: expected bytes, memoryview found

I thinks this is worth fixing. Is there an issue already?

We can just use this one - it was more in the nature of a question "is there anything we want to change about the status quo?" than a request for any specific change.

I'm actually OK with buffer API based interoperability, but if we're going to offer that, we should be consistent:

1. bytes and bytearray should interoperate with anything supporting the buffer interface (which they already mostly do)
2. When they encounter each other, LHS wins (as with set() and frozenset())
3. We should fix the operand coercion bug for C level concatenation slot implementations (already covered by issue #11477) 
4. Update the documentation as needed

Since we're tinkering with builtin behaviour, 1 & 2 should probably be brought up on python-dev once someone checks if there is anything other than .join() that needs updating.

An issue with bytes.join() is already fixed (issue15958).

Reproduced on 3.11.