How do you read from stdin

Question

I m trying to do some of the code golf challenges  but they all require the input to be taken from stdin  How do I get that in Python

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When using -c command  as a tricky way  instead of reading the stdin  and more flexible in some cases  you can pass a shell script command as well to your python command by putting the shell command in quotes within a parenthesis started by   sign  e g  python3 -c  quot import sys  print len sys argv 1  split   n     quot   quot   cat    goldendict history  quot   This will count the number of lines from goldendict s history file

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You could use the fileinput module  import fileinput  for line in fileinput input        pass  fileinput will loop through all the lines in the input specified as file names given in command-line arguments  or the standard input if no arguments are provided  Note  line will contain a trailing newline  to remove it use line rstrip

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There is  os read 0  x  which reads xbytes from 0 which represents stdin  This is an unbuffered read  more low level than sys stdin read

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This will echo standard input to standard output   import sys line   sys stdin readline   while line      print line      line   sys stdin readline

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For Python 3 that would be     Filename e g  cat py import sys  for line in sys stdin      print line  end       This is basically a simple form of cat 1   since it doesn t add a newline after each line  You can use this  after You marked the file executable using chmod  x cat py such as   echo Hello     cat py

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The problem I have with solution  import sys  for line in sys stdin      print line    is that if you don t pass any data to stdin  it will block forever  That s why I love this answer  check if there is some data on stdin first  and then read it  This is what I ended up doing   import sys import select    select files to read from  files to write to  magic  timeout    timeout 0 0 is essential b c we want to know the asnwer right away if select select  sys stdin           0 0  0       help file fragment   sys stdin read   else      print  No data passed to stdin   file sys stderr      sys exit 2

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I had some issues when getting this to work for reading over sockets piped to it  When the socket got closed it started returning empty string in an active loop  So this is my solution to it  which I only tested in linux  but hope it works in all other systems   import sys  os sep os linesep  while sep    os linesep      data   sys stdin readline                      sep   data -len os linesep        print   gt    s     data strip     So if you start listening on a socket it will work properly  e g  in bash    while    do nc -l 12345   python test py   done   And you can call it with telnet or just point a browser to localhost 12345

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I use the following method  it returns a string from stdin  I use it for json parsing   It works with pipe and prompt on Windows  not tested on Linux yet   When prompting  two line breaks indicate end of input   def get from stdin       lb   0   stdin         for line in sys stdin      if line      n           lb    1         if lb    2              break     else          lb   0         stdin    line    return stdin

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Building on all the anwers using sys stdin  you can also do something like the following to read from an argument file if at least one argument exists  and fall back to stdin otherwise   import sys f   open sys argv 1   if len sys argv   gt  1 else sys stdin     for line in f        Do your stuff   and use it as either    python do-my-stuff py infile txt   or    cat infile txt   python do-my-stuff py   or even    python do-my-stuff py  lt  infile txt   That would make your Python script behave like many GNU Unix programs such as cat  grep and sed

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Read from sys stdin  but to read binary data on Windows  you need to be extra careful  because sys stdin there is opened in text mode and it will corrupt  r n replacing them with  n   The solution is to set mode to binary if Windows   Python 2 is detected  and on Python 3 use sys stdin buffer   import sys  PY3K   sys version info  gt    3  0   if PY3K      source   sys stdin buffer else        Python 2 on Windows opens sys stdin in text mode  and       binary data that read from it becomes corrupted on  r n     if sys platform     win32             set sys stdin to binary mode         import os  msvcrt         msvcrt setmode sys stdin fileno    os O BINARY      source   sys stdin  b   source read

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The answer proposed by others   for line in sys stdin    print line   is very simple and pythonic  but it must be noted that the script will wait until EOF before starting to iterate on the lines of input   This means that tail -f error log   myscript py will not process lines as expected   The correct script for such a use case would be   while 1      try          line   sys stdin readline       except KeyboardInterrupt          break      if not line          break      print line   UPDATE From the comments it has been cleared that on python 2 only there might be buffering involved  so that you end up waiting for the buffer to fill or EOF before the print call is issued

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argparse is an easy solution  Example compatible with both Python versions 2 and 3      usr bin python  import argparse import sys  parser   argparse ArgumentParser    parser add argument  infile                       default sys stdin                      type argparse FileType  r                        nargs       args   parser parse args    data   args infile read     You can run this script in many ways   1  Using stdin  echo  foo bar      above-script py    emsp  or shorter by replacing echo by here string     above-script py  lt  lt  lt   foo bar    2  Using a filename argument  echo  foo bar   gt  my-file data   above-script py my-file data   3  Using stdin through the special filename -  echo  foo bar      above-script py -

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Here s from Learning Python   import sys data   sys stdin readlines   print  Counted   len data    lines       On Unix  you could test it by doing something like     cat countlines py   python countlines py  Counted 3 lines    On Windows or DOS  you d do   C   gt  type countlines py   python countlines py  Counted 3 lines

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How do you read from stdin in Python  I m trying to do some of the code golf challenges  but they all require the input to be taken from stdin  How do I get that in Python   You can use   sys stdin - A file-like object - call sys stdin read   to read everything  input prompt  - pass it an optional prompt to output  it reads from stdin up to the first newline  which it strips  You d have to do this repeatedly to get more lines  at the end of the input it raises EOFError   Probably not great for golfing   In Python 2  this is rawinput prompt   open 0  read   - In Python 3  the builtin function open accepts file descriptors  integers representing operating system IO resources   and 0 is the descriptor of stdin  It returns a file-like object like sys stdin - probably your best bet for golfing  In Python 2  this is io open  open   dev stdin   read   - similar to open 0   works on Python 2 and 3  but not on Windows  or even Cygwin   fileinput input   - returns an iterator over lines in all files listed in sys argv 1    or stdin if not given  Use like    join fileinput input      Both sys and fileinput must be imported  respectively  of course  Quick sys stdin examples compatible with Python 2 and 3  Windows  Unix You just need to read from sys stdin  for example  if you pipe data to stdin    echo foo   python -c  quot import sys  print sys stdin read    quot  foo  We can see that sys stdin is in default text mode   gt  gt  gt  import sys  gt  gt  gt  sys stdin  lt  io TextIOWrapper name   lt stdin gt   mode  r  encoding  UTF-8  gt   file example Say you have a file  inputs txt  we can accept that file and write it back out  python -c  quot import sys  sys stdout write sys stdin read    quot   lt  inputs txt  Longer answer Here s a complete  easily replicable demo  using two methods  the builtin function  input  use raw input in Python 2   and sys stdin  The data is unmodified  so the processing is a non-operation  To begin with  let s create a file for inputs    python -c  quot print  foo nbar nbaz   quot   gt  inputs txt  And using the code we ve already seen  we can check that we ve created the file    python -c  quot import sys  sys stdout write sys stdin read    quot   lt  inputs txt  foo bar baz  Here s the help on sys stdin read from Python 3  read size -1     method of  io TextIOWrapper instance     Read at most n characters from stream           Read from underlying buffer until we have n characters or we hit EOF      If n is negative or omitted  read until EOF   Builtin function  input  raw input in Python 2  The builtin function input reads from standard input up to a newline  which is stripped  complementing print  which adds a newline by default   This occurs until it gets EOF  End Of File   at which point it raises EOFError  Thus  here s how you can use input in Python 3  or raw input in Python 2  to read from stdin - so we create a Python module we call stdindemo py    python -c  quot print  try  n    while True  n        print input    nexcept EOFError  n    pass   quot   gt  stdindemo py   And let s print it back out to ensure it s as we expect    python -c  quot import sys  sys stdout write sys stdin read    quot   lt  stdindemo py  try      while True          print input    except EOFError      pass  Again  input reads up until the newline and essentially strips it from the line  print adds a newline  So while they both modify the input  their modifications cancel   So they are essentially each other s complement   And when input gets the end-of-file character  it raises EOFError  which we ignore and then exit from the program  And on Linux Unix  we can pipe from cat    cat inputs txt   python -m stdindemo foo bar baz  Or we can just redirect the file from stdin    python -m stdindemo  lt  inputs txt  foo bar baz  We can also execute the module as a script    python stdindemo py  lt  inputs txt  foo bar baz  Here s the help on the builtin input from Python 3  input prompt None         Read a string from standard input   The trailing newline is stripped           The prompt string  if given  is printed to standard output without a     trailing newline before reading input           If the user hits EOF   nix  Ctrl-D  Windows  Ctrl-Z Return   raise EOFError      On  nix systems  readline is used if available   sys stdin Here we make a demo script using sys stdin  The efficient way to iterate over a file-like object is to use the file-like object as an iterator  The complementary method to write to stdout from this input is to simply use sys stdout write    python -c  quot print  import sys nfor line in sys stdin  n    sys stdout write line    quot   gt  stdindemo2 py  Print it back out to make sure it looks right    python -c  quot import sys  sys stdout write sys stdin read    quot   lt  stdindemo2 py  import sys for line in sys stdin      sys stdout write line   And redirecting the inputs into the file    python -m stdindemo2  lt  inputs txt foo bar baz  Golfed into a command    python -c  quot import sys  sys stdout write sys stdin read    quot   lt  inputs txt foo bar baz  File Descriptors for Golfing Since the file descriptors for stdin and stdout are 0 and 1 respectively  we can also pass those to open in Python 3  not 2  and note that we still need the  w  for writing to stdout   If this works on your system  it will shave off more characters    python -c  quot open 1  w   write open 0  read    quot   lt  inputs txt baz bar foo  Python 2 s io open does this as well  but the import takes a lot more space    python -c  quot from io import open  open 1  w   write open 0  read    quot   lt  inputs txt  foo bar baz  Addressing other comments and answers One comment suggests    join sys stdin  for golfing but that s actually longer than sys stdin read   - plus Python must create an extra list in memory  that s how str join works when not given a list  - for contrast     join sys stdin  sys stdin read    The top answer suggests  import fileinput  for line in fileinput input        pass  But  since sys stdin implements the file API  including the iterator protocol  that s just the same as this  import sys  for line in sys stdin      pass  Another answer does suggest this  Just remember that if you do it in an interpreter  you ll need to do Ctrl-d if you re on Linux or Mac  or Ctrl-z on Windows  after Enter  to send the end-of-file character to the process  Also  that answer suggests print line  - which adds a   n  to the end - use print line  end     instead  if in Python 2  you ll need from   future   import print function   The real use-case for fileinput is for reading in a series of files

User · Answer

import sys  for line in sys stdin      print line    Note that this will include a newline character at the end  To remove the newline at the end  use line rstrip   as  brittohalloran said

User · Answer

There s a few ways to do it    sys stdin is a file-like object on which you can call functions read or readlines if you want to read everything or you want to read everything and split it by newline automatically   You need to import sys for this to work   If you want to prompt the user for input  you can use raw input in Python 2 X  and just input in Python 3    If you actually just want to read command-line options  you can access them via the sys argv list    You will probably find this Wikibook article on I O in Python to be a useful reference as well

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The following chip of code will help you  it will read all of stdin blocking unto EOF  into one string    import sys input str   sys stdin read   print input str split

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You can read from stdin and then store inputs into  data  as follows   data      for line in sys stdin      data    line

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Try this   import sys  print sys stdin read   upper     and check it with     echo  Hello World    python myFile py

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I am pretty amazed no one had mentioned this hack so far   python -c  import sys  set map sys stdout write sys stdin      in python2 you can drop the set   call  but it would word either way

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Regarding this   for line in sys stdin   I just tried it on python 2 7  following someone else s suggestion  for a very large file  and I don t recommend it  precisely for the reasons mentioned above  nothing happens for a long time     I ended up with a slightly more pythonic solution  and it works on bigger files    with open sys argv 1    r   as f      for line in f    Then I can run the script locally as   python myscript py  0 1 2 3 4       can be a multi-line string or filename - any std in input will work

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Python also has built-in functions input   and raw input    See the Python documentation under Built-in Functions   For example   name   raw input  Enter your name         Python 2 x   or  name   input  Enter your name         Python 3

[python] How do you read from stdin?

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