Python: check if named pipe has data

Question

I have a Python3 process on my Unix system that always works, and I want to be able to randomly send data through a named pipe from other processes that run only occasionally. If the named pipe has no data, I want my process to continue to do other things, so I need to check if it has data without blocking .

I cannot figure out how to check without opening it, but opening blocks, if I did not set the flag without blocking. And if I set the flag, it will work if I write to the pipe before or during reading.

This is the best I have managed to do:

import os

fifo = "pipe_test.fifo"
done = False
fd = os.open(fifo, os.O_RDONLY | os.O_NONBLOCK)
while not done:
    try:
        s = os.read(fd, 1024) # buffer size may need tweaking
        print(s)
        done = True
    except BlockingIOError as e:
        pass
os.close(fd)

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Answer 1

, , UNIX , :

import errno, fcntl, os, socket

:

# bind socket
sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
sock.bind('pipe_test.fifo')
# set socket non-blocking
fcntl.fcntl(sock.fileno(), fcntl.F_SETFL, os.O_NONBLOCK)

# get a datagram
try:
    datagram = sock.recv(1024)
except (OSError, socket.error) as ex:
    if ex.errno not in (errno.EINTR, errno.EAGAIN):
        raise
else:
    print('Datagram: %r' % datagram)

:

sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
sock.sendto('Hello!', 'pipe_test.fifo')

multithreading .

Answer 2

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class QueryThread(threading.Thread):

    def __init__(self, args=(), kwargs=None):
        threading.Thread.__init__(self, args=(), kwargs=None)
        self.daemon = True
        self.buf = []
        if not general.f_exists("pipe"):
            os.mkfifo("pipe")

    def run(self):
        f = open("pipe")
        while True:
            try:
                query = next(f).replace("\n", "")
                if query != "":
                    self.buf.append(query)
                    print("Read in new query from pipe: {}, buf = {}".format(query, self.buf))
            except StopIteration: # not a pipe error, just means no data is left, so time to re-open
                f.close()
                f = open("pipe")
        f.close()

    def get_query(self):
        if len(self.buf) == 0: return ""
        query = self.buf[0]
        self.buf.__delitem__(0)
        return query

. get_query , .