There are several ways to write to stderr:
print >> sys.stderr, "spam" # Python 2 only. sys.stderr.write("spam\n") os.write(2, b"spam\n") from __future__ import print_function print("spam", file=sys.stderr) What are the differences between these methods? Which method should be preferred?
I found this to be the only one short, flexible, portable and readable:
import sys def eprint(*args, **kwargs): print(*args, file=sys.stderr, **kwargs) The optional function eprint saves some repetition. It can be used in the same way as the standard print function:
>>> print("Test") Test >>> eprint("Test") Test >>> eprint("foo", "bar", "baz", sep="---") foo---bar---baz 
eprint as a class method, don't forget to add self as an argument: def eprint(self, *args, **kwargs):. Otherwise you'll get your output messed up and it might take you a while to find out why :-)
print('your message', file=sys.stderr) now that python2 is dead since years. See Joachim's answer below and upvote that if you like this cleaner solution.
eprint("error", file=sys.stderr) which cause "TypeError: print() got multiple values for keyword argument 'file'". To prevent this you could remove the file=sys.stderr, argument and instead before the print line add the line kwargs['file'] = sys.stderr to ensure the argument is only specified once. I ran into this so you may want to make that change.
print with eprint but assuming you want to do that, it does not seem like a good idea to silently override and ignore a previously existing file argument.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).
print statement is easy printing of non-string values, without having to convert them first. If you need a print statement, I would therefore recommend using the 3rd option to be python 3 ready
sys.stderr.write() is nothing like print. It doesn't add a newline.
os.linesep. This is stderr we're talking about, after all. Don't want the console to mess up with the wrong newline.Python 3:
print("fatal error", file=sys.stderr) Python 2:
print >> sys.stderr, "fatal error" 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.
print('spam', file=sys.stderr). If you're doing it over and over again, you can code the 'eprint' function like in the most popular answer, but in that case, I would ask, what's wrong with logging? stackoverflow.com/a/41304513/1450294
with sys.stderr as dest: before an indented call to print("ERROR", file=dest)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.
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'}))
"{0}".format({'spam': 'spam'}) anyway, wouldn't it? I would say you should avoid explicitly converting to string. Edit: I accidentally a grammar
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) 
>> mean syntactically? I understand that it's an effort to copy bash's >, so is it some shoehorned syntax to do just that?
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") 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.
This will mimic the standard print function but output on stderr
def print_err(*args): sys.stderr.write(' '.join(map(str,args)) + '\n') 
print doesn't include a flush.
print is buffered, but includes a parameter to flush. If buffering is causing problems, it's probably best to use python -u to disable buffering.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.
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:
You may later decide to switch between stdout/stderr and a regular file.
print() syntax has changed in Python 3, so if you need to support both versions, write() might be better.
from __future__ import print_function is a better way to support both Python 2.6+ and Python 3.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 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.
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!
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
Another way
import sys print("{}".format(sys.exec_info()[1], file=sys.stderr) Im doing this just for fun but here is another way... :-)
message = 'error: Belly up!!' print(message, file=sys.stderr if 'error' in message.lower() else sys.stdout) 