Use python's pty to create a live console

Question

I'm trying to create an execution environment/shell that will remotely execute on a server, which streams the stdout,err,in over the socket to be rendered in a browser. I currently have tried the approach of using subprocess.run with a PIPE. The Problem is that I get the stdout after the process has completed. What i want to achieve is to get a line-by-line, pseudo-terminal sort of implementation.

My current implementation

test.py

def greeter():
    for _ in range(10):
        print('hello world')

greeter()

and in the shell

>>> import subprocess
>>> result = subprocess.run(['python3', 'test.py'], stdout=subprocess.PIPE)
>>> print(result.stdout.decode('utf-8'))
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world
hello world

If i try to attempt even this simple implementation with pty, how does one do it?

Answer 1

If your application is going to work asynchronously with multiple tasks, like reading data from stdout and then writing it to a websocket, I suggest using asyncio.

Here is an example that runs a process and redirects its output into a websocket:

import asyncio.subprocess
import os

from aiohttp.web import (Application, Response, WebSocketResponse, WSMsgType,
                         run_app)


async def on_websocket(request):
    # Prepare aiohttp's websocket...
    resp = WebSocketResponse()
    await resp.prepare(request)
    # ... and store in a global dictionary so it can be closed on shutdown
    request.app['sockets'].append(resp)

    process = await asyncio.create_subprocess_exec(sys.executable,
                                                   '/tmp/test.py',
                                                    stdout=asyncio.subprocess.PIPE,
                                                    stderr=asyncio.subprocess.PIPE,
                                                    bufsize=0)
    # Schedule reading from stdout and stderr as asynchronous tasks.
    stdout_f = asyncio.ensure_future(p.stdout.readline())
    stderr_f = asyncio.ensure_future(p.stderr.readline())

    # returncode will be set upon process's termination.
    while p.returncode is None:
        # Wait for a line in either stdout or stderr.
        await asyncio.wait((stdout_f, stderr_f), return_when=asyncio.FIRST_COMPLETED)

        # If task is done, then line is available.
        if stdout_f.done():
            line = stdout_f.result().encode()
            stdout_f = asyncio.ensure_future(p.stdout.readline())
            await ws.send_str(f'stdout: {line}')

        if stderr_f.done():
            line = stderr_f.result().encode()
            stderr_f = asyncio.ensure_future(p.stderr.readline())
            await ws.send_str(f'stderr: {line}')

    return resp


async def on_shutdown(app):
    for ws in app['sockets']:
        await ws.close()    


async def init(loop):
    app = Application()
    app['sockets'] = []
    app.router.add_get('/', on_websocket)
    app.on_shutdown.append(on_shutdown)
    return app


loop = asyncio.get_event_loop()
app = loop.run_until_complete(init())
run_app(app)

It uses aiohttp and is based on the web_ws and subprocess streams examples.