repl in python with popen and selectors

popen-selectors-repl.py

# unfortunately, the stdinready event fires all the time, not just when repl ready.
# Based on https://gist.github.com/andy0130tw/39472331530d1a0e25459a547ed2c9d5

import io
import selectors
from selectors import EVENT_READ, EVENT_WRITE
import subprocess
from typing import cast

sel = selectors.DefaultSelector()

command = ["python", "-i"]
proc = subprocess.Popen(command, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

stdoutbuf = io.BytesIO()  # if you wanted to collect stdout
assert proc.stdin is not None
sel.register(proc.stdin, EVENT_WRITE)
assert proc.stdout is not None
sel.register(proc.stdout, EVENT_READ)

while True:
    gotoutput = False
    wantsinput = False
    for output, _mask in sel.select(timeout=0.6):
        f = output.fileobj
        if f is proc.stdout:
            f = cast(io.BufferedReader, f)
            data = f.read1()  # read1 is very important!!
            # if not data:
            #     continue
            gotoutput = True
            decoded = data.decode("utf-8", errors="replace")
            print(decoded, end="", flush=True)
            stdoutbuf.write(data)
        elif f is proc.stdin:
            wantsinput = True
            # f = cast(io.BufferedWriter, f)
        else:
            raise Exception("unreachable")

    if gotoutput:
        continue
    if wantsinput:
        command = input(" ! ")
        # print(f"{command=}")
        proc.stdin.write(command.encode("utf-8") + b"\n")
        proc.stdin.flush()
        continue
    print("see ya")
    break
print("all done")