Non-blocking pipe reads in Windows Python

NonBlockingPipe.md

This is code that was born while working on https://stackoverflow.com/questions/34504970/non-blocking-read-on-os-pipe-on-windows

It consists of ospipen.py module with a sigle function:

def pipe_no_wait(pipefd):
  """ pipefd is a integer as returned by os.pipe """

ossub.py that tests the concept on subprocess.PIPE, and Go program for that subprocess.

Everything is in public domain.

osgo.go

package main

import (
	"fmt"
	"time"
)

func main() {
	fmt.Print("Heya! Two seconds..\n")
	time.Sleep(2000 * time.Millisecond)
	fmt.Print("Heya! One more..\n")
	time.Sleep(1000 * time.Millisecond)
	fmt.Print("Heya! Done.")
}

ospipen.py

import msvcrt
import os

from ctypes import windll, byref, wintypes, GetLastError, WinError
from ctypes.wintypes import HANDLE, DWORD, POINTER, BOOL

LPDWORD = POINTER(DWORD)

PIPE_NOWAIT = wintypes.DWORD(0x00000001)

ERROR_NO_DATA = 232

def pipe_no_wait(pipefd):
  """ pipefd is a integer as returned by os.pipe """

  SetNamedPipeHandleState = windll.kernel32.SetNamedPipeHandleState
  SetNamedPipeHandleState.argtypes = [HANDLE, LPDWORD, LPDWORD, LPDWORD]
  SetNamedPipeHandleState.restype = BOOL

  h = msvcrt.get_osfhandle(pipefd)

  res = windll.kernel32.SetNamedPipeHandleState(h, byref(PIPE_NOWAIT), None, None)
  if res == 0:
      print(WinError())
      return False
  return True


if __name__  == '__main__':
  # CreatePipe
  r, w = os.pipe()

  pipe_no_wait(r)

  print os.write(w, 'xxx')
  print os.read(r, 1024)
  try:
    print os.write(w, 'yyy')
    print os.read(r, 1024)
    print os.read(r, 1024)
  except OSError as e:
    print dir(e), e.errno, GetLastError()
    print(WinError())
    if GetLastError() != ERROR_NO_DATA:
        raise

  print os.write(w, 'zzz')
  print os.read(r, 1024)

ossub.py

import subprocess

"""
p = subprocess.Popen('python.exe -c "import time; print(\\"xxx\\"); time.sleep(2); print(\\"yyy\\")', stdout = subprocess.PIPE)
print p.stdout
print p.stdout.fileno()
print p.stdout.read()"""
# ^ blocking read, reads xxx\nyyy after 2 seconds. probably due to buffering


from ospipen import pipe_no_wait

#p = subprocess.Popen('python.exe -c "import time; print(\\"xxx\\"); time.sleep(2); print(\\"yyy\\")', stdout = subprocess.PIPE)
p = subprocess.Popen('osgo.exe', stdout = subprocess.PIPE)
pipe_no_wait(p.stdout.fileno())
print p.stdout
print p.stdout.fileno()
import time
while True:
  try:
    print ".", p.stdout.read(), ".", time.time(), p.stdout.closed, p.poll()
  except IOError as e:
    print e
    time.sleep(0.1)

In Python3 LPDWORD is already defined, so I changed ospipen.py from:

from ctypes.wintypes import HANDLE, DWORD, POINTER, BOOL

LPDWORD = POINTER(DWORD)

to:

from ctypes.wintypes import HANDLE, DWORD, LPDWORD, BOOL

Good to know. As I don't use Windows anymore, feel free to fork it and place a link here so that people know where to find maintained version.

Nice solution, you might have just saved me few days..:)