[python] How to print to stderr in Python?

There are several ways to write to stderr:

# Note: this first one does not work in Python 3
print >> sys.stderr, "spam"

sys.stderr.write("spam\n")

os.write(2, b"spam\n")

from __future__ import print_function
print("spam", file=sys.stderr)

That seems to contradict zen of Python #13 ^†, so what's the difference here and are there any advantages or disadvantages to one way or the other? Which way should be used?

^† _{There should be one — and preferably only one — obvious way to do it.}

In Python 3, one can just use print():

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

almost out of the box:

import sys
print("Hello, world!", file=sys.stderr)

or:

from sys import stderr
print("Hello, world!", file=stderr)

This is straightforward and does not need to include anything besides sys.stderr.

I am working in python 3.4.3. I am cutting out a little typing that shows how I got here:

[18:19 jsilverman@JSILVERMAN-LT7 pexpect]$ python3
>>> import sys
>>> print("testing", file=sys.stderr)
testing
>>>
[18:19 jsilverman@JSILVERMAN-LT7 pexpect]$ 

Did it work? Try redirecting stderr to a file and see what happens:

[18:22 jsilverman@JSILVERMAN-LT7 pexpect]$ python3 2> /tmp/test.txt
>>> import sys
>>> print("testing", file=sys.stderr)
>>> [18:22 jsilverman@JSILVERMAN-LT7 pexpect]$
[18:22 jsilverman@JSILVERMAN-LT7 pexpect]$ cat /tmp/test.txt
Python 3.4.3 (default, May  5 2015, 17:58:45)
[GCC 4.9.2] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
testing

[18:22 jsilverman@JSILVERMAN-LT7 pexpect]$

Well, aside from the fact that the little introduction that python gives you has been slurped into stderr (where else would it go?), it works.

If you do a simple test:

import time
import sys

def run1(runs):
    x = 0
    cur = time.time()
    while x < runs:
        x += 1
        print >> sys.stderr, 'X'
    elapsed = (time.time()-cur)
    return elapsed

def run2(runs):
    x = 0
    cur = time.time()
    while x < runs:
        x += 1
        sys.stderr.write('X\n')
        sys.stderr.flush()
    elapsed = (time.time()-cur)
    return elapsed

def compare(runs):
    sum1, sum2 = 0, 0
    x = 0
    while x < runs:
        x += 1
        sum1 += run1(runs)
        sum2 += run2(runs)
    return sum1, sum2

if __name__ == '__main__':
    s1, s2 = compare(1000)
    print "Using (print >> sys.stderr, 'X'): %s" %(s1)
    print "Using (sys.stderr.write('X'),sys.stderr.flush()):%s" %(s2)
    print "Ratio: %f" %(float(s1) / float(s2))

You will find that sys.stderr.write() is consistently 1.81 times faster!

Python 2:

print >> sys.stderr, "fatal error"

Python 3:

print("fatal error", file=sys.stderr)

Long answer

print >> sys.stderr is gone in Python3. http://docs.python.org/3.0/whatsnew/3.0.html says:

Old: print >> sys.stderr, "fatal error"
New: print("fatal error", file=sys.stderr)

For many of us, it feels somewhat unnatural to relegate the destination to the end of the command. The alternative

sys.stderr.write("fatal error\n")

looks more object oriented, and elegantly goes from the generic to the specific. But note that write is not a 1:1 replacement for print.

EDIT In hind-sight, I think the potential confusion with changing sys.stderr and not seeing the behaviour updated makes this answer not as good as just using a simple function as others have pointed out.

Using partial only saves you 1 line of code. The potential confusion is not worth saving 1 line of code.

original

To make it even easier, here's a version that uses 'partial', which is a big help in wrapping functions.

from __future__ import print_function
import sys
from functools import partial

error = partial(print, file=sys.stderr)

You then use it like so

error('An error occured!')

You can check that it's printing to stderr and not stdout by doing the following (over-riding code from http://coreygoldberg.blogspot.com.au/2009/05/python-redirect-or-turn-off-stdout-and.html):

# over-ride stderr to prove that this function works.
class NullDevice():
    def write(self, s):
        pass
sys.stderr = NullDevice()

# we must import print error AFTER we've removed the null device because
# it has been assigned and will not be re-evaluated.
# assume error function is in print_error.py
from print_error import error

# no message should be printed
error("You won't see this error!")

The downside to this is partial assigns the value of sys.stderr to the wrapped function at the time of creation. Which means, if you redirect stderr later it won't affect this function. If you plan to redirect stderr, then use the **kwargs method mentioned by aaguirre on this page.

I did the following using Python 3:

from sys import stderr

def print_err(*args, **kwargs):
    print(*args, file=stderr, **kwargs)

So now I'm able to add keyword arguments, for example, to avoid carriage return:

print_err("Error: end of the file reached. The word ", end='')
print_err(word, "was not found")

The same applies to stdout:

print 'spam'
sys.stdout.write('spam\n')

As stated in the other answers, print offers a pretty interface that is often more convenient (e.g. for printing debug information), while write is faster and can also be more convenient when you have to format the output exactly in certain way. I would consider maintainability as well:

1. You may later decide to switch between stdout/stderr and a regular file.

2. print() syntax has changed in Python 3, so if you need to support both versions, write() might be better.

import sys
sys.stderr.write()

Is my choice, just more readable and saying exactly what you intend to do and portable across versions.

Edit: being 'pythonic' is a third thought to me over readability and performance... with these two things in mind, with python 80% of your code will be pythonic. list comprehension being the 'big thing' that isn't used as often (readability).

I would say that your first approach:

print >> sys.stderr, 'spam' 

is the "One . . . obvious way to do it" The others don't satisfy rule #1 ("Beautiful is better than ugly.")

-- Edit for 2020 --

Above was my answer for Python 2.7 in 2011. Now that Python 3 is the standard, I think the "right" answer is:

print("spam", file=sys.stderr) 

If you want to exit a program because of a fatal error, use:

sys.exit("Your program caused a fatal error. ... description ...")

and import sys in the header.

This will mimic the standard print function but output on stderr

def print_err(*args):
    sys.stderr.write(' '.join(map(str,args)) + '\n')

Nobody's mentioned logging yet, but logging was created specifically to communicate error messages. Basic configuration will set up a stream handler writing to stderr.

This script:

# foo.py
import logging

logging.basicConfig(format='%(message)s')
log = logging.getLogger(__name__)
log.warning('I print to stderr by default')
print('hello world')

has the following result when run on the command line:

$ python3 foo.py > bar.txt
I print to stderr by default

and bar.txt will contain the 'hello world' printed on stdout.

Answer to the question is : There are different way to print stderr in python but that depends on 1.) which python version we are using 2.) what exact output we want.

The differnce between print and stderr's write function: stderr : stderr (standard error) is pipe that is built into every UNIX/Linux system, when your program crashes and prints out debugging information (like a traceback in Python), it goes to the stderr pipe.

print: print is a wrapper that formats the inputs (the input is the space between argument and the newline at the end) and it then calls the write function of a given object, the given object by default is sys.stdout, but we can pass a file i.e we can print the input in a file also.

Python2: If we are using python2 then

>>> import sys
>>> print "hi"
hi
>>> print("hi")
hi
>>> print >> sys.stderr.write("hi")
hi

Python2 trailing comma has in Python3 become a parameter, so if we use trailing commas to avoid the newline after a print, this will in Python3 look like print('Text to print', end=' ') which is a syntax error under Python2.

http://python3porting.com/noconv.html

If we check same above sceario in python3:

>>> import sys
>>> print("hi")
hi

Under Python 2.6 there is a future import to make print into a function. So to avoid any syntax errors and other differences we should start any file where we use print() with from future import print_function. The future import only works under Python 2.6 and later, so for Python 2.5 and earlier you have two options. You can either convert the more complex print to something simpler, or you can use a separate print function that works under both Python2 and Python3.

>>> from __future__ import print_function
>>> 
>>> def printex(*args, **kwargs):
...     print(*args, file=sys.stderr, **kwargs)
... 
>>> printex("hii")
hii
>>>

Case: Point to be noted that sys.stderr.write() or sys.stdout.write() ( stdout (standard output) is a pipe that is built into every UNIX/Linux system) is not a replacement for print, but yes we can use it as a alternative in some case. Print is a wrapper which wraps the input with space and newline at the end and uses the write function to write. This is the reason sys.stderr.write() is faster.

Note: we can also trace and debugg using Logging

#test.py
import logging
logging.info('This is the existing protocol.')
FORMAT = "%(asctime)-15s %(clientip)s %(user)-8s %(message)s"
logging.basicConfig(format=FORMAT)
d = {'clientip': '192.168.0.1', 'user': 'fbloggs'}
logging.warning("Protocol problem: %s", "connection reset", extra=d)

https://docs.python.org/2/library/logging.html#logger-objects

For Python 2 my choice is: print >> sys.stderr, 'spam' Because you can simply print lists/dicts etc. without convert it to string. print >> sys.stderr, {'spam': 'spam'} instead of: sys.stderr.write(str({'spam': 'spam'}))