How to print to stderr in Python

Question

There are several ways to write to stderr     Note  this first one does not work in Python 3 print  gt  gt  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

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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

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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

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I would say that your first approach  print  gt  gt  sys stderr   spam    is the  quot One       obvious way to do it quot   The others don t satisfy rule  1   quot Beautiful is better than ugly  quot   -- Edit for 2020 -- Above was my answer for Python 2 7 in 2011  Now that Python 3 is the standard  I think the  quot right quot  answer is  print  quot spam quot   file sys stderr

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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

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For Python 2 my choice is  print  gt  gt  sys stderr   spam  Because you can simply print lists dicts etc  without convert it to string   print  gt  gt  sys stderr    spam    spam   instead of  sys stderr write str   spam    spam

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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  gt  gt  gt  import sys  gt  gt  gt  print  testing   file sys stderr  testing  gt  gt  gt   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 gt   tmp test txt  gt  gt  gt  import sys  gt  gt  gt  print  testing   file sys stderr   gt  gt  gt   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

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Python 2  print  gt  gt  sys stderr   quot fatal error quot   Python 3  print  quot fatal error quot   file sys stderr    Long answer print  gt  gt  sys stderr is gone in Python3  http   docs python org 3 0 whatsnew 3 0 html says   Old  print  gt  gt  sys stderr   quot fatal error quot   New  print  quot fatal error quot   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  quot fatal error n quot    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

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I found this to be the only one short   flexible   portable   readable   from   future   import print function import sys  def eprint  args    kwargs       print  args  file sys stderr    kwargs    The function eprint can be used in the same way as the standard print function    gt  gt  gt  print  Test   Test  gt  gt  gt  eprint  Test   Test  gt  gt  gt  eprint  foo    bar    baz   sep  ---   foo---bar---baz

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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

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This will mimic the standard print function but output on stderr  def print err  args       sys stderr write     join map str args       n

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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  gt  bar txt I print to stderr by default   and bar txt will contain the  hello world  printed on stdout

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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

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If you do a simple test   import time import sys  def run1 runs       x   0     cur   time time       while x  lt  runs          x    1         print  gt  gt  sys stderr   X      elapsed    time time  -cur      return elapsed  def run2 runs       x   0     cur   time time       while x  lt  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  lt  runs          x    1         sum1    run1 runs          sum2    run2 runs      return sum1  sum2  if   name         main         s1  s2   compare 1000      print  Using  print  gt  gt  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

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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

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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   gt  gt  gt  import sys  gt  gt  gt  print  hi  hi  gt  gt  gt  print  hi   hi  gt  gt  gt  print  gt  gt  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     gt  gt  gt  import sys  gt  gt  gt  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     gt  gt  gt  from   future   import print function  gt  gt  gt    gt  gt  gt  def printex  args    kwargs           print  args  file sys stderr    kwargs        gt  gt  gt  printex  hii   hii  gt  gt  gt       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

[python] How to print to stderr in Python?

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