Python open fifo blocks forever

Question

I'm trying to implement IPC using named pipes in Python, but there is a problem. open blocks the process

import os
path = '/tmp/fifo'
os.mkfifo(path)
fifo = open(path, 'w') # never returns

Same with open(path, 'r')

What am I doing wrong?

Python 3.6.1

Answer 1

Figured this out. open blocks until the pipe is open on the other side

With os.open, the statement will be nonblocking if os.O_NONBLOCK is given as a flag. Nonblocking may not work on your OS. I believe it works on Unix distros but not Windows.

Answer 2

Correction to the accpeted answer, which is too inaccurate, if not totally wrong. Below is from man fifo(7) of Linux:

First, blocking mode opening. This is what OP found:

Normally, opening the FIFO blocks until the other end is opened also.

Non-blocking, read-only opening case is:

A process can open a FIFO in nonblocking mode. In this case, opening for read-only succeeds even if no one has opened on the write side yet

But non-blocking, write-only opening is dangerous:

and opening for write-only fails with ENXIO (no such device or address) unless the other end has already been opened.

There is a workaround, namely read-write opening. But this also needs care:

Under Linux, opening a FIFO for read and write will succeed both in blocking and nonblocking mode. POSIX leaves this behavior undefined. This can be used to open a FIFO for writing while there are no readers available.

A practical caution follows in the manual page :)

A process that uses both ends of the connection in order to communicate with itself should be, very careful to avoid deadlocks.