Remove unused _complete_fut attribute from asyncio.StreamWriter

[deadlovelll]

Bug report

Bug description:

asyncio.StreamWriter.__init__ creates a _complete_fut Future and stores it on the instance:

# Lib/asyncio/streams.py
def __init__(self, transport, protocol, reader, loop):
    self._transport = transport
    self._protocol = protocol
    assert reader is None or isinstance(reader, StreamReader)
    self._reader = reader
    self._loop = loop
    self._complete_fut = self._loop.create_future()
    self._complete_fut.set_result(None)

The StreamWriter class is used by functios such as: asyncio.open_connection, asyncio.start_server, asyncio.open_unix_connection, asyncio.start_unix_server. So the allocation of an unused future happens on every asyncio network connection

The attribute is not read anywhere in CPython. The project search returned only two lines in StreamWriter.__init__

Proposed change:

Drop the two _complete_fut lines:

def __init__(self, transport, protocol, reader, loop):
    self._transport = transport
    self._protocol = protocol
    assert reader is None or isinstance(reader, StreamReader)
    self._reader = reader
    self._loop = loop

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-150358 Remove unused _complete_fut attribute from asyncio.StreamWriter #150359

[picnixz]

Can you dig into git history to understand why we had this attribute for posterity?

[deadlovelll]

Can you dig into git history to understand why we had this attribute for posterity?

Hello!

This attribute was originally added to be used in _fast_drain(). commit

Then, by the commit 23b4b69, stream classes were merged into a unified one. _fast_drain() and _complete_fut were also moved here

But the second commit was reverted, it restored the initialization of _complete_fut in StreamWriter, but did not restore _fast_drain(). So the await write() API introduced by the first commit was dropped together with the stream merge revert.