Getting a BlockingIOError when printing or writting to stdout

Question

I am getting the little documented and less present in the web "BlockingIOError" when writing to the tty from a program where I need the terminal to be in "raw" mode.

Switching the terminal to raw in Unix (Linux, Mac) is the way to get lines to show without printing a newline, and, more important, to read whatever is typed without having to wait for the <enter> key.

Check https://stackoverflow.com/a/6599441/108205 for a reliable way to do it in Python.

However, when printing data to the raw terminal, I would, at random times, have my program terminate with a BlockingIOError exception.

(I had even built a retry mechanism to workaround it, but sometimes it was not work-aroundable at all).

The exception shows up in some issues on github, with no assertion on how to fix it. The way to trigger it with my project is:

import terminedia as TM

with TM.keyboard:  # enables the use of TM.inkey() for realtime keyboard reading
    print("*" * 100_000)

(you can pip install terminedia to try)

the fix/workaround on the accepted answer is now on the project, and used in a seamless way for all output. Install the old version terminedia==0.2.1 to be able to trigger the issue as in the question text. — jsbueno, Jan 13 '22 at 12:53

score 2 · Accepted Answer · answered May 02 '21 at 00:12

This snippet, creating a context manager which momentarily switches the tty back to "normal" blocking mode wrapping the parts that have a lot of output fixed the issue for me:

import fcntl
import os
...

class UnblockTTY:

    def __enter__(self):
        self.fd = sys.stdin.fileno()
        self.flags_save = fcntl.fcntl(self.fd, fcntl.F_GETFL)
        flags = self.flags_save & ~os.O_NONBLOCK
        fcntl.fcntl(self.fd, fcntl.F_SETFL, flags)

    def __exit__(self, *args):
        fcntl.fcntl(self.fd, fcntl.F_SETFL, self.flags_save)

For completeness, the snippet in the question would be fixed with:

import terminedia as TM

with TM.keyboard:  
    with TM.terminal.UnblockTTY():
        print("*" * 100_000)

(although when using terminedia one should be using the "Screen" class rather or TM.print which are already wrapped)

If unblocked, won't the write call return that not all bytes were written and some of the output will be lost? — user48956, Nov 10 '22 at 17:49
Maybe the name is reversed in the code there: the blocking mode is what works - Unblocking mode raises the exception. — jsbueno, Nov 10 '22 at 17:57

Getting a BlockingIOError when printing or writting to stdout

1 Answers1