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IOError: [Errno 32] Broken pipe: Python

Posted by: admin November 30, 2017 Leave a comment

Questions:

I have a very simple Python 3 script:

f1 = open('a.txt', 'r')
print(f1.readlines())
f2 = open('b.txt', 'r')
print(f2.readlines())
f3 = open('c.txt', 'r')
print(f3.readlines())
f4 = open('d.txt', 'r')
print(f4.readlines())
f1.close()
f2.close()
f3.close()
f4.close()

But it always says:

IOError: [Errno 32] Broken pipe

I saw on the internet all the complicated ways to fix this, but I copied this code directly, so I think that there is something wrong with the code and not Python’s SIGPIPE.

I am redirecting the output, so if the above script was named “open.py”, then my command to run would be:

open.py | othercommand
Answers:

I haven’t reproduced the issue, but perhaps this method would solve it: (writing line by line to stdout rather than using print)

import sys
with open('a.txt', 'r') as f1:
    for line in f1:
        sys.stdout.write(line)

You could catch the broken pipe? This writes the file to stdout line by line until the pipe is closed.

import sys, errno
try:
    with open('a.txt', 'r') as f1:
        for line in f1:
            sys.stdout.write(line)
except IOError as e:
    if e.errno == errno.EPIPE:
        # Handle error

You also need to make sure that othercommand is reading from the pipe before it gets too big – https://unix.stackexchange.com/questions/11946/how-big-is-the-pipe-buffer

Questions:
Answers:

The problem is due to SIGPIPE handling. You can solve this problem using the following code:

from signal import signal, SIGPIPE, SIG_DFL
signal(SIGPIPE,SIG_DFL) 

See here for background on this solution.

Questions:
Answers:

To bring Alex L.’s helpful answer, akhan’s helpful answer, and Blckknght’s helpful answer together with some additional information:

  • Standard Unix signal SIGPIPE is sent to a process writing to a pipe when there’s no process reading from the pipe (anymore).

    • This is not necessarily an error condition; some Unix utilities such as head by design stop reading prematurely from a pipe, once they’ve received enough data.
  • By default – i.e., if the writing process does not explicitly trap SIGPIPE – the writing process is simply terminated, and its exit code is set to 141, which is calculated as 128 (to signal termination by signal in general) + 13 (SIGPIPE‘s specific signal number).

  • By design, however, Python itself traps SIGPIPE and translates it into a Python IOError instance with errno value errno.EPIPE, so that a Python script can catch it, if it so chooses – see Alex L.’s answer for how to do that.

  • If a Python script does not catch it, Python outputs error message IOError: [Errno 32] Broken pipe and terminates the script with exit code 1 – this is the symptom the OP saw.

  • In many cases this is more disruptive than helpful, so reverting to the default behavior is desirable:

    • Using the signal module allows just that, as stated in akhan’s answer; signal.signal() takes a signal to handle as the 1st argument and a handler as the 2nd; special handler value SIG_DFL represents the system’s default behavior:

      from signal import signal, SIGPIPE, SIG_DFL
      signal(SIGPIPE, SIG_DFL) 
      
Questions:
Answers:

A “Broken Pipe” error occurs when you try to write to a pipe that has been closed on the other end. Since the code you’ve shown doesn’t involve any pipes directly, I suspect you’re doing something outside of Python to redirect the standard output of the Python interpreter to somewhere else. This could happen if you’re running a script like this:

python foo.py | someothercommand

The issue you have is that someothercommand is exiting without reading everything available on its standard input. This causes your write (via print) to fail at some point.

I was able to reproduce the error with the following command on a Linux system:

python -c 'for i in range(1000): print i' | less

If I close the less pager without scrolling through all of its input (1000 lines), Python exits with the same IOError you have reported.

Questions:
Answers:

I feel obliged to point out that the method using

signal(SIGPIPE, SIG_DFL) 

is indeed dangerous (as already suggested by David Bennet in the comments) and in my case led to platform-dependent funny business when combined with multiprocessing.Manager (because the standard library relies on BrokenPipeError being raised in several places). To make a long and painful story short, this is how I fixed it:

First, you need to catch the IOError (Python 2) or BrokenPipeError (Python 3). Depending on your program you can try to exit early at that point or just ignore the exception:

from errno import EPIPE

try:
    broken_pipe_exception = BrokenPipeError
except NameError:  # Python 2
    broken_pipe_exception = IOError

try:
    YOUR CODE GOES HERE
except broken_pipe_exception as exc:
    if broken_pipe_exception == IOError:
        if exc.errno != EPIPE:
            raise

However, this isn’t enough. Python 3 may still print a message like this:

Exception ignored in: <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>
BrokenPipeError: [Errno 32] Broken pipe

Unfortunately getting rid of that message is not straightforward, but I finally found http://bugs.python.org/issue11380 where Robert Collins suggests this workaround that I turned into a decorator you can wrap your main function with (yes, that’s some crazy indentation):

from functools import wraps
from sys import exit, stderr, stdout
from traceback import print_exc


def suppress_broken_pipe_msg(f):
    @wraps(f)
    def wrapper(*args, **kwargs):
        try:
            return f(*args, **kwargs)
        except SystemExit:
            raise
        except:
            print_exc()
            exit(1)
        finally:
            try:
                stdout.flush()
            finally:
                try:
                    stdout.close()
                finally:
                    try:
                        stderr.flush()
                    finally:
                        stderr.close()
    return wrapper


@suppress_broken_pipe_msg
def main():
    YOUR CODE GOES HERE

Questions:
Answers:

This can also occur if the read end of the output from your script dies prematurely

ie open.py | otherCommand

if otherCommand exits and open.py tries to write to stdout

I had a bad gawk script that did this lovely to me.

Questions:
Answers:

Closes should be done in reverse order of the opens.