To prevent a function from printing in the batch console in Python

Question

Well, the headline seems to me sufficient. I use some function that at some points print something in the console. As I can't modify them, I would like to know if there is a solution to not printing while using these functions.

Thanks a lot !

Nico

Answer 1

Yes, you can redirect sys.stdout:

import sys
import os

old_stdout = sys.stdout # backup current stdout
sys.stdout = open(os.devnull, "w")

my_nasty_function()

sys.stdout = old_stdout # reset old stdout

Just replace my_nasty_function with your actual function.

EDIT: Now should work on windows aswell.

EDIT: Use backup variable to reset stdout is better when someone wraps your function again

Answer 2

Constantinius' answer answer is ok, however there is no need to actually open null device. And BTW, if you want portable null device, there is os.devnull.

Actually, all you need is a class which will ignore whatever you write to it. So more portable version would be:

class NullIO(StringIO):
    def write(self, txt):
       pass

sys.stdout = NullIO()

my_nasty_function()

sys.stdout = sys.__stdout__

.

Answer 3

Another option would be to wrap your function in a decorator.

from contextlib import redirect_stdout
from io import StringIO 

class NullIO(StringIO):
    def write(self, txt):
        pass


def silent(fn):
    """Decorator to silence functions."""
    def silent_fn(*args, **kwargs):
        with redirect_stdout(NullIO()):
            return fn(*args, **kwargs)
    return silent_fn


def nasty():
    """Useful function with nasty prints."""
    print('a lot of annoying output')
    return 42


# Wrap in decorator to prevent printing.
silent_nasty = silent(nasty)
# Same output, but prints only once.
print(nasty(), silent_nasty())

Answer 4

You could use a modified version of this answer to create a "null" output context to wrap the call the function in.

That can be done by just passing os.devnull as the new_stdout argument to the stdout_redirected() context manager function when it's used.

Answer 5

The currently accepted answer by Constantinius works great in most circumstances, but not in Jupyter notebooks. Here's how to get it to work (with a reusable function)...

TLDR~Instead of using sys.__stout__, backup sys.stdout and restore it later on.

In a Jupyter notebook, running sys.stdout == sys.__stdout__ returns false. This is because each cell has a separate output stream (instead of the one terminal instance, which is sys.__stdout__). So for everyone working with Jupyter notebooks, make sure to back up the old sys.stdout path and restore it afterwards. Here's a function for it:

import sys, os
def deafen(function, *args):
    real_stdout = sys.stdout
    sys.stdout = open(os.devnull, "w")
    output = function(*args)
    sys.stdout = real_stdout
    return output

Pass to deafen a function along with its arguments/parameters (args). It backs up the old sys.stdout, switches to os.devnull and back again itself.

For a complete example we can create a second function (test_function):

def test_function(first_argument, second_argument, *args):
    print(first_argument)
    print(second_argument)
    print(args)

Now if we try using the test_function like normal (a.k.a. without deafen) we will get a bunch of output printed onto the screen:

print("Printing should work fine here")
test_function(32, 12, 1, 32, 1)

However, when using deafen, we'll get no new output:

print("That'll be the last thing you see...")
deafen(test_function, 32, 12, 1, 32, 1)

On a side note, the deafen function still returns a functions output. You can also use deafen with sys.__stdout__ by replacing sys.stdout = real_stdout with sys.stdout = sys.__stdout__ (and may as well remove real_stdout = sys.stdout whilst you're at it).

Hope that helps anyone who is looking for a slightly more robust or flexible solution (likely for Jupyter notebooks, or use with multiple functions)!

Answer 6

Constantinius' solution will work on *nix, but this should work on any platform:

import sys
import tempfile

sys.stdout = tempfile.TemporaryFile()

# Do crazy stuff here

sys.stdout.close()
#now the temp file is gone
sys.stdout = sys.__stdout__