[Python-ideas] Add function readbyte to asyncio.StreamReader

Jörn Heissler python-ideas-2018 at tutnicht.de
Fri Jul 27 03:48:51 EDT 2018

On Mon, Jul 23, 2018 at 22:25:14 +0100, Gustavo Carneiro wrote:
> Well, even if it is worth, i.e. your use case is not rare enough,

Reading a single byte is certainly a special case, but I think it's generic
enough to warrant its own function.
I believe there should be many binary protocols out there that would benefit
from such a function.

For example the first socks5 (rfc1928) message could be parsed like this:

async def read_methods(reader):
    if (version := await reader.readbyte()) != 5:
        raise Exception(f'Bad version: {version}')
    if (nmethods := await reader.readbyte()) == 0:
        raise Exception('Need at least one method')
    return await reader.readexactly(nmethods)

> I would
> suggest at least making it private, readexactly can call this specialised
> function if nbytes==1:
> def _readbyte(self):
>    ....
> def readexactly(self, num):
>    if num == 1:
>       return self._readbyte()
>   ... the rest stays the same..

Maybe I wasn't clear in my intent: readbyte would not return a bytes object but
an integer, i.e. the byte value.

My current approach is this:

value = (await reader.readexactly(1))[0]

I'd like to make it more readable (and faster at the same time):

value = await reader.readbyte()

> But to be honest, you are probably better off managing the buffer yourself:
> Just call, e.g., stream.read(4096), it will return a buffer of up to 4k
> length, then you can iterate over the buffer byte by byte until the
> condition is met, repeat until the end of stream, or whatever.

StreamReader already manages a buffer. Managing a second buffer would
mean I'd need to copy all my data from one buffer to another.

But let's assume I went this way and iterated over my own buffer:

* I receive some bytes. Maybe it's exactly the amount I need, then I can parse
  it and discard the buffer.
* Or it's less than I need. I'd have to wait for more data and either restart my
  parser or remember the state from before.
* Or it's more than I need. I'd have to remove the parsed bytes from the buffer.
  Alternatively I could push back the unparsed bytes to my buffer.

This adds lots of code complexity. And the code is likely slower than calling
readbyte() couple of times; for my current use case, calling it once is usually

I like my current approach way better than managing my own buffer and thus
reinventing StreamReader.

Adding the new function as proposed would improve the code in both readability
and speed.

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