Can I redirect the stdout in python into some sort of string buffer

Question

I m using python s ftplib to write a small FTP client  but some of the functions in the package don t return string output  but print to stdout  I want to redirect stdout to an object which I ll be able to read the output from   I know stdout can be redirected into any regular file with   stdout   open  file    a     But I prefer a method that doesn t uses the local drive   I m looking for something like the BufferedReader in Java that can be used to wrap a buffer into a stream

User · Answer

This method restores sys stdout even if there s an exception   It also gets any output before the exception   import io import sys  real stdout   sys stdout fake stdout   io BytesIO       or perhaps io StringIO   try      sys stdout   fake stdout       do what you have to do to create some output finally      sys stdout   real stdout     output string   fake stdout getvalue       fake stdout close         do what you want with the output string   Tested in Python 2 7 10 using  io BytesIO    Tested in Python 3 6 4 using io StringIO      Bob  added for a case if you feel anything from the modified   extended code experimentation might get interesting in any sense  otherwise feel free to delete it     Ad informandum     a few remarks from extended experimentation during finding some viable mechanics to  grab  outputs  directed by numexpr print versions   directly to the  lt stdout gt    upon a need to clean GUI and collecting details into debugging-report       THIS WORKS AS HELL  as Bob Stein proposed years ago     py2 SURPRISEDaBIT    import io import sys   real stdout   sys stdout                                    PUSH  lt stdout gt    store to REAL    fake stdout   io BytesIO                                     DEF FAKE  try                                               FUSED  TRY      sys stdout flush                                         flush   before     sys stdout   fake stdout                                 SET  lt stdout gt  to use FAKE        -----------------------------------------                  do what you gotta do to create some output     print 123456789                                                import  numexpr                                                QuantFX numexpr   version                                  3  via fake stdout re-assignment  as was bufferred    late  deferred  get value  -read into print  to finally reach - gt  real stdout     QuantFX numexpr print versions                             4  via fake stdout re-assignment  as was bufferred    late  deferred  get value  -read into print  to finally reach - gt  real stdout         os system   echo os system   redir-ed                  1  via real stdout                                    late  deferred  get value  -read into print  to finally reach - gt  real stdout  if not        -caught from RET-d  byteswritten    avoided from being injected int fake stdout         os write   sys stderr fileno                           2  via      stderr                                    late  deferred  get value  -read into print  to finally reach - gt  real stdout  if not        -caught from RET-d  byteswritten    avoided from being injected int fake stdout                        b os write    redir-ed       OTHERWISE  if via fake stdout  EXC  lt  io BytesIO object at 0x02C0BB10 gt  Traceback  most recent call last         -----------------------------------------                                            io UnsupportedOperation  fileno                                                               YET          lt  io BytesIO object at 0x02C0BB10 gt  has a  fileno   method listed       gt  gt  gt   fileno  in dir  sys stdout         - gt  True          HAS IT ADVERTISED        gt  gt  gt  pass             sys stdout fileno  - gt   lt built-in method fileno of  io BytesIO object at 0x02C0BB10 gt        gt  gt  gt  pass             sys stdout fileno  - gt  Traceback  most recent call last                                                     File   lt stdin gt    line 1  in  lt module gt                                                  io UnsupportedOperation  fileno                                                               BUT REFUSES TO USE IT          finally                                              FINALLY      sys stdout flush                                         flush   before ret d back REAL      sys stdout   real stdout                                 SET  lt stdout gt  to use POP d REAL      sys stdout flush                                         flush   after  ret d back REAL      out string   fake stdout getvalue                        GET string           from FAKE      fake stdout close                                             lt FD gt  close                                                     do what you want with the out string           print   n 0   n 1   0    format  60            LATE  deferred print the out string at the very end reached - gt  real stdout                                      out string                                                                 PASS d          os system   redir-ed os write    redir-ed                                                                                                                          123456789  2 5  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  Numexpr version    2 5 NumPy version      1 10 4 Python version     2 7 13  Anaconda 4 0 0  32-bit    default  May 11 2017  14 07 41   MSC v 1500 32 bit  Intel   AMD Intel CPU      True VML available      True VML MKL version    Intel R  Math Kernel Library Version 11 3 1 Product Build 20151021 for 32-bit applications Number of threads used by default  4  out of 4 detected cores  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -                                                                                                                            gt  gt  gt   EXC d           os system   redir-ed                                                                                                                          123456789  2 5  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  Numexpr version    2 5 NumPy version      1 10 4 Python version     2 7 13  Anaconda 4 0 0  32-bit    default  May 11 2017  14 07 41   MSC v 1500 32 bit  Intel   AMD Intel CPU      True VML available      True VML MKL version    Intel R  Math Kernel Library Version 11 3 1 Product Build 20151021 for 32-bit applications Number of threads used by default  4  out of 4 detected cores  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -                                                                                                                            Traceback  most recent call last     File   lt stdin gt    line 9  in  lt module gt  io UnsupportedOperation  fileno

User · Answer

Just to add to Ned s answer above  you can use this to redirect output to any object that implements a write str  method   This can be used to good effect to  catch  stdout output in a GUI application   Here s a silly example in PyQt   import sys from PyQt4 import QtGui  class OutputWindow QtGui QPlainTextEdit       def write self  txt           self appendPlainText str txt    app   QtGui QApplication sys argv  out   OutputWindow   sys stdout out out show   print  hello world

User · Answer

Starting with Python 2 6 you can use anything implementing the TextIOBase API from the io module as a replacement  This solution also enables you to use sys stdout buffer write   in Python 3 to write  already  encoded byte strings to stdout  see stdout in Python 3   Using StringIO wouldn t work then  because neither sys stdout encoding nor sys stdout buffer would be available   A solution using TextIOWrapper   import sys from io import TextIOWrapper  BytesIO    setup the environment old stdout   sys stdout sys stdout   TextIOWrapper BytesIO    sys stdout encoding     do something that writes to stdout or stdout buffer    get output sys stdout seek 0         jump to the start out   sys stdout read     read output    restore stdout sys stdout close   sys stdout   old stdout   This solution works for Python 2    2 6 and Python 3   Please note that our new sys stdout write   only accepts unicode strings and sys stdout buffer write   only accepts byte strings  This might not be the case for old code  but is often the case for code that is built to run on Python 2 and 3 without changes  which again often makes use of sys stdout buffer   You can build a slight variation that accepts unicode and byte strings for write     class StdoutBuffer TextIOWrapper       def write self  string           try              return super StdoutBuffer  self  write string          except TypeError                redirect encoded byte strings directly to buffer             return super StdoutBuffer  self  buffer write string    You don t have to set the encoding of the buffer the sys stdout encoding  but this helps when using this method for testing comparing script output

User · Answer

A context manager for python3   import sys from io import StringIO   class RedirectedStdout      def   init   self           self  stdout   None         self  string io   None      def   enter   self           self  stdout   sys stdout         sys stdout   self  string io   StringIO           return self      def   exit   self  type  value  traceback           sys stdout   self  stdout      def   str   self           return self  string io getvalue     use like this    gt  gt  gt  with RedirectedStdout   as out   gt  gt  gt      print  asdf    gt  gt  gt      s   str out   gt  gt  gt      print  bsdf    gt  gt  gt  print s  out   asdf n   asdf nbsdf n

User · Answer

There is contextlib redirect stdout   function in Python 3 4   import io from contextlib import redirect stdout  with io StringIO   as buf  redirect stdout buf       print  redirected       output   buf getvalue     Here s code example that shows how to implement it on older Python versions

User · Answer

Here s another take on this   contextlib redirect stdout with io StringIO   as documented is great  but it s still a bit verbose for every day use   Here s how to make it a one-liner by subclassing contextlib redirect stdout   import sys import io from contextlib import redirect stdout  class capture redirect stdout        def   init   self           self f   io StringIO           self  new target   self f         self  old targets         verbatim from parent class      def   enter   self           self  old targets append getattr sys  self  stream      verbatim from parent class         setattr sys  self  stream  self  new target     verbatim from parent class         return self    instead of self  new target in the parent class      def   repr   self           return self f getvalue       Since   enter   returns self  you have the context manager object available after the with block exits   Moreover  thanks to the   repr   method  the string representation of the context manager object is  in fact  stdout   So now you have   with capture   as message      print  Hello World    print str message    Hello World  n      returns True

User · Answer

Use pipe   and write to the appropriate file descriptor   https   docs python org library os html file-descriptor-operations

User · Answer

In Python3 6  the StringIO and cStringIO modules are gone  you should use io StringIO instead So you should do this like the first answer   import sys from io import StringIO  old stdout   sys stdout old stderr   sys stderr my stdout   sys stdout   StringIO   my stderr   sys stderr   StringIO      blah blah lots of code      sys stdout   self old stdout sys stderr   self old stderr     if you want to see the value of redirect output  be sure the std output is turn back print my stdout getvalue    print my stderr getvalue     my stdout close   my stderr close

User · Answer

from cStringIO import StringIO   Python3 use  from io import StringIO import sys  old stdout   sys stdout sys stdout   mystdout   StringIO      blah blah lots of code      sys stdout   old stdout    examine mystdout getvalue

[python] Can I redirect the stdout in python into some sort of string buffer?

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