Cross platform /dev/null in Python

Question

I'm using the following code to hide stderr on Linux/OSX for a Python library I do not control that writes to stderr by default:

f = open("/dev/null","w")
zookeeper.set_log_stream(f)

Is there an easy cross platform alternative to /dev/null? Ideally it would not consume memory since this is a long running process.

possible duplicate of [/dev/null in Windows?](http://stackoverflow.com/questions/313111/dev-null-in-windows) — msw, May 28 '10 at 14:39
@msw: I don't think so, Python has more ways you can deal with this issue. — Andrew Aylett, May 28 '10 at 14:45

score 162 · Accepted Answer · answered May 28 '10 at 14:40

162

How about os.devnull ?

import os
f = open(os.devnull,"w")
zookeeper.set_log_stream(f)

answered May 28 '10 at 14:40

msanders

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score 48 · Answer 2 · answered May 28 '10 at 14:40

48

class Devnull(object):
    def write(self, *_): pass

zookeeper.set_log_stream(Devnull())

Opening os.devnull is fine too of course, but this way every output operation occurs (as a noop) "in process" -- no context switch to the OS and back, and also no buffering (while some buffering is normally used by an open) and thus even less memory consumption.

answered May 28 '10 at 14:40

Alex Martelli

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I understand that using os.devnull may produce some overhead. But if one uses your object what if the zookeeper object calls other methods then `write` of its log_stream file object? Maybe it calls the `writelines` method? Then there is an exception. – miracle173 Apr 05 '14 at 09:50
5

This doesn't work when you need a *real* file, e.g. one with `fileno()`. – Jonathon Reinhart Apr 07 '15 at 22:48
@JonathonReinhart For that, I suppose you could create the file descriptor lazily on request, using `os.open(os.devnull, os.O_RDWR)` and yielding the same fd for subsequent calls to `fileno` (since all data are discarded anyway) – minmaxavg Aug 29 '17 at 10:08
You'll also need `close()` – shoosh Feb 20 '19 at 09:19
@shoosh Would inhering from `io.IOBase` and just overwrite `write()` as above do the trick? That would provide all the methods clients might expect. – joanis Dec 06 '21 at 21:51

score 6 · Answer 3 · answered May 28 '10 at 14:40

6

>>> import os
>>> os.devnull
'nul'

answered May 28 '10 at 14:40

SilentGhost

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Just to clarify: 'nul' is given on Windows. Linux will return '/dev/null'. – Walter May 28 '10 at 14:42

Andrew Aylett · Answer 4 · 2010-05-29T09:42:16.680

5

Create your own file-like object which doesn't do anything?

class FakeSink(object):
    def write(self, *args):
        pass
    def writelines(self, *args):
        pass
    def close(self, *args):
        pass

edited May 29 '10 at 09:42

answered May 28 '10 at 14:42

Andrew Aylett

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Idiomatically, you're right, but 'self' is just another parameter and will be passed in as the first element of `args`. As we don't use any of the parameters, the only reason to care is aesthetic. I'll fix it... – Andrew Aylett May 29 '10 at 09:41
2

Some operations also need 'fileno' – Jonathan Hartley May 27 '15 at 09:17

badp · Answer 5 · 2010-05-28T14:57:09.630

Cheap solution warning!

class DevNull():
  def __init__(self, *args):
    self.closed = False
    self.mode = "w"
    self.name = "<null>"
    self.encoding = None
    self.errors = None
    self.newlines = None
    self.softspace = 0
  def close(self):
    self.closed == True
  @open_files_only
  def flush(self):
    pass
  @open_files_only
  def next(self):
    raise IOError("Invalid operation")
  @open_files_only
  def read(size = 0):
    raise IOError("Invalid operation")
  @open_files_only
  def readline(self):
    raise IOError("Invalid operation")
  @open_files_only
  def readlines(self):
    raise IOError("Invalid operation")
  @open_files_only
  def xreadlines(self):
    raise IOError("Invalid operation")
  @open_files_only
  def seek(self):
    raise IOError("Invalid operation")
  @open_files_only
  def tell(self):
    return 0
  @open_files_only
  def truncate(self):
    pass
  @open_files_only
  def write(self):
    pass
  @open_files_only
  def writelines(self):
    pass

def open_files_only(fun):
  def wrapper(self, *args):
    if self.closed:
      raise IOError("File is closed")
    else:
      fun(self, *args)
  return wrapper

I threw in a decorator just for fun :D – badp May 28 '10 at 14:52 — badp, May 28 '10 at 14:52
Also need __enter__ and __exit__ ? – user48956 Feb 11 '16 at 18:28 — user48956, Feb 11 '16 at 18:28

Cross platform /dev/null in Python

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