Implicit bytes and str conversion

[nmattia]

I was trying to figure out why asyncio.Stream.write accepts a str as a buffer (hoping this is the correct category). It looks like a str can be appended to a bytes:

>>> b'foo' + 'bar'
b'foobar'

The converse however is not true:

>>> 'bar' + b'foo'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported types for __add__: 'str', 'bytes'

(at least on the unix port).

Does asyncio.Stream.write officially support str? And is + : bytes -> str -> bytes expected?

[nmattia]

Looks like this was already reported, at least for the implicit conversion when concatenating: #3856

[GitHubsSilverBullet]

>>> 'bar' + b'foo'

You have a string (bar) and you want to append bytes(foo)
The question is: what's in those bytes? How to interpret them? ASCII, UTF-8?
'bar' + b'foo'.decode('utf8')
works just fine.

[peterhinch]

The bug at issue is that

b'foo' + 'bar'

throws an exception in CPython, but is accepted by MicroPython.

[nmattia]

Made worse by the fact that

MicroPython strives to be a more regular implementation, so if both str and bytes support mod() (the % operator), it makes sense to support format() for both too.
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