use socket from exec_run in python3 (python2 is working) -- 'SocketIO' object has no attribute 'sendall'

The following code works fine with pyhton2 (docker-py 2.5.1), but not with python3 (docker-py 3.7.0)

import docker
  
client = docker.DockerClient(base_url='unix://var/run/docker.sock', version='auto')
container = client.containers.run("gliderlabs/alpine", command= "sleep 100", detach = True)
socket = container.exec_run(cmd="sh", stdin=True, socket = True)
socket.sendall(b"ls\n")

# a read block after a send
try:
    unknown_byte=socket.recv(1)
    while 1:
        # note that os.read does not work
        # because it does not TLS-decrypt
        # but returns the low-level encrypted data
        # one must use "socket.recv" instead
        data = socket.recv(16384)
        if not data: break
        print(data.decode('utf8'))
except so.timeout: pass

socket.sendall(b"exit\n")

in python3 "container.exec_run" returns ExecResult(exit_code=None, output=<socket.SocketIO object at 0x7f5bb7f7c160>).

So my idea was to use socket.output.sendall(b "ls\n"), but then I always get the error AttributeError: 'SocketIO' object has no attribute 'sendall'"".
The documentation says only If socket=True, a socket object for the connection, and sendall() is a socket method for me.

How to use exec_run in python3?

Source:
https://github.com/docker/docker-py/issues/983
https://stackoverflow.com/questions/46521166/how-to-write-to-stdin-in-a-tls-enabled-docker-using-the-python-docker-api
https://github.com/mailcow/mailcow-dockerized/pull/2297

Version:
docker-host:
{'Platform': {'Name': ''}, 'Components': [{'Name': 'Engine', 'Version': '18.03.1-ce', 'Details': {'ApiVersion': '1.37', 'Arch': 'amd64', 'BuildTime': '2018-04-26T07:15:30.000000000+00:00', 'Experimental': 'false', 'GitCommit': '9ee9f40', 'GoVersion': 'go1.9.5', 'KernelVersion': '4.15.0-45-generic', 'MinAPIVersion': '1.12', 'Os': 'linux'}}], 'Version': '18.03.1-ce', 'ApiVersion': '1.37', 'MinAPIVersion': '1.12', 'GitCommit': '9ee9f40', 'GoVersion': 'go1.9.5', 'Os': 'linux', 'Arch': 'amd64', 'KernelVersion': '4.15.0-45-generic', 'BuildTime': '2018-04-26T07:15:30.000000000+00:00'}

conatiner
Alpine 3.9 with docker-py 3.7.0

Start at the container.exec_run entry point and reproduce the Python 3 case with socket=True using the example in the issue. Trace the returned ExecResult output and socket handling, then verify that interactive stdin supports the documented send and receive workflow without the SocketIO AttributeError.