fifo - reading in a loop

Question

I want to use os.mkfifo for simple communication between programs. I have a problem with reading from the fifo in a loop.

Consider this toy example, where I have a reader and a writer working with the fifo. I want to be able to run the reader in a loop to read everything that enters the fifo.

# reader.py
import os
import atexit

FIFO = 'json.fifo'

@atexit.register
def cleanup():
    try:
        os.unlink(FIFO)
    except:
        pass

def main():
    os.mkfifo(FIFO)
    with open(FIFO) as fifo:
#        for line in fifo:              # closes after single reading
#        for line in fifo.readlines():  # closes after single reading
        while True:
            line = fifo.read()          # will return empty lines (non-blocking)
            print repr(line)

main()

And the writer:

# writer.py
import sys

FIFO = 'json.fifo'


def main():
    with open(FIFO, 'a') as fifo:
        fifo.write(sys.argv[1])

main()

If I run python reader.py and later python writer.py foo, "foo" will be printed but the fifo will be closed and the reader will exit (or spin inside the while loop). I want reader to stay in the loop, so I can execute the writer many times.

Edit

I use this snippet to handle the issue:

def read_fifo(filename):
    while True:
        with open(filename) as fifo:
            yield fifo.read()

but maybe there is some neater way to handle it, instead of repetitively opening the file...

Related

• Getting readline to block on a FIFO

Answer 1

You do not need to reopen the file repeatedly. You can use select to block until data is available.

with open(FIFO_PATH) as fifo:
    while True:
        select.select([fifo],[],[fifo])
        data = fifo.read()
        do_work(data)

In this example you won't read EOF.