How to flush output of print function

Question

How do I force Python s print function to output to the screen   This is not a duplicate of Disable output buffering - the linked question is attempting unbuffered output  while this is more general  The top answers in that question are too powerful or involved for this one  they re not good answers for this   and this question can be found on Google by a relative newbie

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With Python 3 x the print   function has been extended   print  objects  sep      end   n   file sys stdout  flush False    So  you can just do   print  Visiting toilet   flush True      Python Docs Entry

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Since Python 3 3  you can force the normal print   function to flush without the need to use sys stdout flush    just set the  flush  keyword argument to true   From the documentation      print  objects  sep      end   n   file sys stdout  flush False       Print objects to the stream file  separated by sep and followed by end  sep  end and file  if present  must be given as keyword arguments       All non-keyword arguments are converted to strings like str   does and written to the stream  separated by sep and followed by end  Both sep and end must be strings  they can also be None  which means to use the default values  If no objects are given  print   will just write end       The file argument must be an object with a write string  method  if it is not present or None  sys stdout will be used  Whether output is buffered is usually determined by file  but if the flush keyword argument is true  the stream is forcibly flushed

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On Python 3  print can take an optional flush argument  print  Hello world    flush True    On Python 2 you ll have to do  import sys sys stdout flush     after calling print  By default  print prints to sys stdout  see the documentation for more about file objects

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Using the -u command-line switch works  but it is a little bit clumsy  It would mean that the program would potentially behave incorrectly if the user invoked the script without the -u option   I usually use a custom stdout  like this   class flushfile    def   init   self  f       self f   f    def write self  x       self f write x      self f flush    import sys sys stdout   flushfile sys stdout        Now all your print calls  which use sys stdout implicitly   will be automatically flushed

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In Python 3 you can overwrite print function with default set to flush   True   def print  objects  sep      end   n   file sys stdout  flush True         builtins   print  objects  sep sep  end end  file file  flush flush

User · Answer

Since Python 3 3  you can force the normal print   function to flush without the need to use sys stdout flush    just set the  flush  keyword argument to true   From the documentation      print  objects  sep      end   n   file sys stdout  flush False       Print objects to the stream file  separated by sep and followed by end  sep  end and file  if present  must be given as keyword arguments       All non-keyword arguments are converted to strings like str   does and written to the stream  separated by sep and followed by end  Both sep and end must be strings  they can also be None  which means to use the default values  If no objects are given  print   will just write end       The file argument must be an object with a write string  method  if it is not present or None  sys stdout will be used  Whether output is buffered is usually determined by file  but if the flush keyword argument is true  the stream is forcibly flushed

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On Python 3  print can take an optional flush argument  print  Hello world    flush True    On Python 2 you ll have to do  import sys sys stdout flush     after calling print  By default  print prints to sys stdout  see the documentation for more about file objects

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Here is my version  which provides writelines   and fileno    too   class FlushFile object       def   init   self  fd           self fd   fd      def write self  x           ret   self fd write x          self fd flush           return ret      def writelines self  lines           ret   self writelines lines          self fd flush           return ret      def flush self           return self fd flush      def close self           return self fd close        def fileno self           return self fd fileno

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On Python 3  print can take an optional flush argument  print  Hello world    flush True    On Python 2 you ll have to do  import sys sys stdout flush     after calling print  By default  print prints to sys stdout  see the documentation for more about file objects

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I did it like this in Python 3 4      To write to screen in real-time    message   lambda x  print x  flush True  end     message  I am flushing out now

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Dan s idea doesn t quite work      usr bin env python class flushfile file       def   init   self  f           self f   f     def write self  x           self f write x          self f flush    import sys sys stdout   flushfile sys stdout   print  foo    The result   Traceback  most recent call last     File    passpersist py   line 12  in  lt module gt      print  foo  ValueError  I O operation on closed file   I believe the problem is that it inherits from the file class  which actually isn t necessary  According to the docs for sys stdout      stdout and stderr needn   t be built-in   file objects  any object is acceptable   as long as it has a write   method   that takes a string argument    so changing  class flushfile file     to  class flushfile object     makes it work just fine

User · Answer

I did it like this in Python 3 4      To write to screen in real-time    message   lambda x  print x  flush True  end     message  I am flushing out now

User · Answer

Using the -u command-line switch works  but it is a little bit clumsy  It would mean that the program would potentially behave incorrectly if the user invoked the script without the -u option   I usually use a custom stdout  like this   class flushfile    def   init   self  f       self f   f    def write self  x       self f write x      self f flush    import sys sys stdout   flushfile sys stdout        Now all your print calls  which use sys stdout implicitly   will be automatically flushed

User · Answer

Why not try using an unbuffered file   f   open  xyz log    a   0    OR  sys stdout   open  out log    a   0

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Dan s idea doesn t quite work      usr bin env python class flushfile file       def   init   self  f           self f   f     def write self  x           self f write x          self f flush    import sys sys stdout   flushfile sys stdout   print  foo    The result   Traceback  most recent call last     File    passpersist py   line 12  in  lt module gt      print  foo  ValueError  I O operation on closed file   I believe the problem is that it inherits from the file class  which actually isn t necessary  According to the docs for sys stdout      stdout and stderr needn   t be built-in   file objects  any object is acceptable   as long as it has a write   method   that takes a string argument    so changing  class flushfile file     to  class flushfile object     makes it work just fine

User · Answer

Running python -h  I see a command line option      -u       unbuffered binary stdout and stderr  also PYTHONUNBUFFERED x            see man page for details on internal buffering relating to  -u    Here is the relevant doc

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Also as suggested in this blog one can reopen sys stdout in unbuffered mode   sys stdout   os fdopen sys stdout fileno     w   0    Each stdout write and print operation will be automatically flushed afterwards

User · Answer

Dan s idea doesn t quite work      usr bin env python class flushfile file       def   init   self  f           self f   f     def write self  x           self f write x          self f flush    import sys sys stdout   flushfile sys stdout   print  foo    The result   Traceback  most recent call last     File    passpersist py   line 12  in  lt module gt      print  foo  ValueError  I O operation on closed file   I believe the problem is that it inherits from the file class  which actually isn t necessary  According to the docs for sys stdout      stdout and stderr needn   t be built-in   file objects  any object is acceptable   as long as it has a write   method   that takes a string argument    so changing  class flushfile file     to  class flushfile object     makes it work just fine

User · Answer

Using the -u command-line switch works  but it is a little bit clumsy  It would mean that the program would potentially behave incorrectly if the user invoked the script without the -u option   I usually use a custom stdout  like this   class flushfile    def   init   self  f       self f   f    def write self  x       self f write x      self f flush    import sys sys stdout   flushfile sys stdout        Now all your print calls  which use sys stdout implicitly   will be automatically flushed

User · Answer

Here is my version  which provides writelines   and fileno    too   class FlushFile object       def   init   self  fd           self fd   fd      def write self  x           ret   self fd write x          self fd flush           return ret      def writelines self  lines           ret   self writelines lines          self fd flush           return ret      def flush self           return self fd flush      def close self           return self fd close        def fileno self           return self fd fileno

User · Answer

Running python -h  I see a command line option      -u       unbuffered binary stdout and stderr  also PYTHONUNBUFFERED x            see man page for details on internal buffering relating to  -u    Here is the relevant doc

User · Answer

I first struggled to understand how the flush option was working  I wanted to do a  loading display  and here is the solution I found   for i in range 100000       print    s  r  format      end     flush True      print  Loading index    d  100000  format i 1   end       The first line flushes the previous print and the second line prints a new updated message  I don t know if an one-line syntax exists here

User · Answer

Also as suggested in this blog one can reopen sys stdout in unbuffered mode   sys stdout   os fdopen sys stdout fileno     w   0    Each stdout write and print operation will be automatically flushed afterwards

User · Answer

Running python -h  I see a command line option      -u       unbuffered binary stdout and stderr  also PYTHONUNBUFFERED x            see man page for details on internal buffering relating to  -u    Here is the relevant doc

User · Answer

Using the -u command-line switch works  but it is a little bit clumsy  It would mean that the program would potentially behave incorrectly if the user invoked the script without the -u option   I usually use a custom stdout  like this   class flushfile    def   init   self  f       self f   f    def write self  x       self f write x      self f flush    import sys sys stdout   flushfile sys stdout        Now all your print calls  which use sys stdout implicitly   will be automatically flushed

User · Answer

I first struggled to understand how the flush option was working  I wanted to do a  loading display  and here is the solution I found   for i in range 100000       print    s  r  format      end     flush True      print  Loading index    d  100000  format i 1   end       The first line flushes the previous print and the second line prints a new updated message  I don t know if an one-line syntax exists here

User · Answer

Dan s idea doesn t quite work      usr bin env python class flushfile file       def   init   self  f           self f   f     def write self  x           self f write x          self f flush    import sys sys stdout   flushfile sys stdout   print  foo    The result   Traceback  most recent call last     File    passpersist py   line 12  in  lt module gt      print  foo  ValueError  I O operation on closed file   I believe the problem is that it inherits from the file class  which actually isn t necessary  According to the docs for sys stdout      stdout and stderr needn   t be built-in   file objects  any object is acceptable   as long as it has a write   method   that takes a string argument    so changing  class flushfile file     to  class flushfile object     makes it work just fine

User · Answer

Running python -h  I see a command line option      -u       unbuffered binary stdout and stderr  also PYTHONUNBUFFERED x            see man page for details on internal buffering relating to  -u    Here is the relevant doc

User · Answer

Why not try using an unbuffered file   f   open  xyz log    a   0    OR  sys stdout   open  out log    a   0

User · Answer

How to flush output of Python print   I suggest five ways of doing this   In Python 3  call print      flush True   the flush argument is not available in Python 2 s print function  and there is no analogue for the print statement   Call file flush   on the output file  we can wrap python 2 s print function to do this   for example  sys stdout apply this to every print function call in the module with a partial function  print   partial print  flush True  applied to the module global  apply this to the process with a flag  -u  passed to the interpreter command apply this to every python process in your environment with PYTHONUNBUFFERED TRUE  and unset the variable to undo this    Python 3 3  Using Python 3 3 or higher  you can just provide flush True as a keyword argument to the print function  print  foo   flush True    Python 2  or  lt  3 3  They did not backport the flush argument to Python 2 7 So if you re using Python 2  or less than 3 3   and want code that s compatible with both 2 and 3  may I suggest the following compatibility code   Note the   future   import must be at very  quot near the top of your module quot    from   future   import print function import sys  if sys version info  2   lt   3  3       old print   print     def print  args    kwargs           flush   kwargs pop  flush   False          old print  args    kwargs          if flush              file   kwargs get  file   sys stdout                Why might file None  IDK  but it works for print i  file None              file flush   if file is not None else sys stdout flush    The above compatibility code will cover most uses  but for a much more thorough treatment  see the six module  Alternatively  you can just call file flush   after printing  for example  with the print statement in Python 2  import sys print  delayed output  sys stdout flush    Changing the default in one module to flush True You can change the default for the print function by using functools partial on the global scope of a module  import functools print   functools partial print  flush True   if you look at our new partial function  at least in Python 3   gt  gt  gt  print   functools partial print  flush True   gt  gt  gt  print functools partial  lt built-in function print gt   flush True   We can see it works just like normal   gt  gt  gt  print  foo   foo  And we can actually override the new default   gt  gt  gt  print  foo   flush False  foo  Note again  this only changes the current global scope  because the print name on the current global scope will overshadow the builtin print function  or unreference the compatibility function  if using one in Python 2  in that current global scope   If you want to do this inside a function instead of on a module s global scope  you should give it a different name  e g   def foo        printf   functools partial print  flush True      printf  print stuff like this    If you declare it a global in a function  you re changing it on the module s global namespace  so you should just put it in the global namespace  unless that specific behavior is exactly what you want  Changing the default for the process I think the best option here is to use the -u flag to get unbuffered output    python -u script py  or   python -um package module  From the docs   Force stdin  stdout and stderr to be totally unbuffered  On systems where it matters  also put stdin  stdout and stderr in binary mode  Note that there is internal buffering in file readlines   and File Objects  for line in sys stdin  which is not influenced by this option  To work around this  you will want to use file readline   inside a while 1  loop   Changing the default for the shell operating environment You can get this behavior for all python processes in the environment or environments that inherit from the environment if you set the environment variable to a nonempty string  e g   in Linux or OSX    export PYTHONUNBUFFERED TRUE  or Windows  C  SET PYTHONUNBUFFERED TRUE  from the docs   PYTHONUNBUFFERED If this is set to a non-empty string it is equivalent to specifying the -u option    Addendum Here s the help on the print function from Python 2 7 12 - note that there is no flush argument   gt  gt  gt  from   future   import print function  gt  gt  gt  help print  print          print value       sep      end   n   file sys stdout           Prints the values to a stream  or to sys stdout by default      Optional keyword arguments      file  a file-like object  stream   defaults to the current sys stdout      sep   string inserted between values  default a space      end   string appended after the last value  default a newline

User · Answer

How to flush output of Python print   I suggest five ways of doing this   In Python 3  call print      flush True   the flush argument is not available in Python 2 s print function  and there is no analogue for the print statement   Call file flush   on the output file  we can wrap python 2 s print function to do this   for example  sys stdout apply this to every print function call in the module with a partial function  print   partial print  flush True  applied to the module global  apply this to the process with a flag  -u  passed to the interpreter command apply this to every python process in your environment with PYTHONUNBUFFERED TRUE  and unset the variable to undo this    Python 3 3  Using Python 3 3 or higher  you can just provide flush True as a keyword argument to the print function  print  foo   flush True    Python 2  or  lt  3 3  They did not backport the flush argument to Python 2 7 So if you re using Python 2  or less than 3 3   and want code that s compatible with both 2 and 3  may I suggest the following compatibility code   Note the   future   import must be at very  quot near the top of your module quot    from   future   import print function import sys  if sys version info  2   lt   3  3       old print   print     def print  args    kwargs           flush   kwargs pop  flush   False          old print  args    kwargs          if flush              file   kwargs get  file   sys stdout                Why might file None  IDK  but it works for print i  file None              file flush   if file is not None else sys stdout flush    The above compatibility code will cover most uses  but for a much more thorough treatment  see the six module  Alternatively  you can just call file flush   after printing  for example  with the print statement in Python 2  import sys print  delayed output  sys stdout flush    Changing the default in one module to flush True You can change the default for the print function by using functools partial on the global scope of a module  import functools print   functools partial print  flush True   if you look at our new partial function  at least in Python 3   gt  gt  gt  print   functools partial print  flush True   gt  gt  gt  print functools partial  lt built-in function print gt   flush True   We can see it works just like normal   gt  gt  gt  print  foo   foo  And we can actually override the new default   gt  gt  gt  print  foo   flush False  foo  Note again  this only changes the current global scope  because the print name on the current global scope will overshadow the builtin print function  or unreference the compatibility function  if using one in Python 2  in that current global scope   If you want to do this inside a function instead of on a module s global scope  you should give it a different name  e g   def foo        printf   functools partial print  flush True      printf  print stuff like this    If you declare it a global in a function  you re changing it on the module s global namespace  so you should just put it in the global namespace  unless that specific behavior is exactly what you want  Changing the default for the process I think the best option here is to use the -u flag to get unbuffered output    python -u script py  or   python -um package module  From the docs   Force stdin  stdout and stderr to be totally unbuffered  On systems where it matters  also put stdin  stdout and stderr in binary mode  Note that there is internal buffering in file readlines   and File Objects  for line in sys stdin  which is not influenced by this option  To work around this  you will want to use file readline   inside a while 1  loop   Changing the default for the shell operating environment You can get this behavior for all python processes in the environment or environments that inherit from the environment if you set the environment variable to a nonempty string  e g   in Linux or OSX    export PYTHONUNBUFFERED TRUE  or Windows  C  SET PYTHONUNBUFFERED TRUE  from the docs   PYTHONUNBUFFERED If this is set to a non-empty string it is equivalent to specifying the -u option    Addendum Here s the help on the print function from Python 2 7 12 - note that there is no flush argument   gt  gt  gt  from   future   import print function  gt  gt  gt  help print  print          print value       sep      end   n   file sys stdout           Prints the values to a stream  or to sys stdout by default      Optional keyword arguments      file  a file-like object  stream   defaults to the current sys stdout      sep   string inserted between values  default a space      end   string appended after the last value  default a newline

User · Answer

On Python 3  print can take an optional flush argument  print  Hello world    flush True    On Python 2 you ll have to do  import sys sys stdout flush     after calling print  By default  print prints to sys stdout  see the documentation for more about file objects

User · Answer

With Python 3 x the print   function has been extended   print  objects  sep      end   n   file sys stdout  flush False    So  you can just do   print  Visiting toilet   flush True      Python Docs Entry

User · Answer

In Python 3 you can overwrite print function with default set to flush   True   def print  objects  sep      end   n   file sys stdout  flush True         builtins   print  objects  sep sep  end end  file file  flush flush

[python] How to flush output of print function?

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