testcase.py

import sys
import neovim
from threading import Thread
from time import sleep
vim = neovim.connect(sys.argv[1])
buf = vim.current.buffer

def thread():
    sleep(0.2)
    for i in range(100):
        sleep(0.001)
        vim.post("test_msg", i)

t = Thread(target=thread)

t.start()
buf = vim.current.buffer
while True:
    kind,name,arg = vim.next_message()
    if name == "test_msg":
        val = buf[-1]
        assert val is not None
        buf[-1:] = [ val, "text"]
        if arg == 99: break