sys stdin readline reads without prompt returning nothing in between

Question

I have a function that executes the following  among other things     userinput   stdin readline   betAmount   int userinput    Is supposed to take input integer from stdin as a string and convert it to an integer   When I call the function  however  it returns a single newline character  it doesn t even wait for me to input anything    Earlier in the program I get some input in the form of   stdin read 1    to capture a single character   Could this have something to do with it  Am I somehow writing a newline character to the next line of stdin   How can I fix this

User · Accepted Answer

stdin read 1  reads one character from stdin  If there was more than one character to be read at that point  e g  the newline that followed the one character that was read in  then that character or characters will still be in the buffer waiting for the next read   or readline      As an example  given rd py   from sys import stdin  x   stdin read 1  userinput   stdin readline   betAmount   int userinput  print   x   x  print   userinput   userinput  print   betAmount   betAmount        if I run this script as follows  I ve typed in the 234    C   gt python rd py 234 x  2 userinput  34  betAmount  34       so the 2 is being picked up first  leaving the 34 and the trailing newline character to be picked up by the readline     I d suggest fixing the problem by using readline   rather than read   under most circumstances

User · Answer

stdin read 1    will not return when you press one character - it will wait for   n   The problem is that the second character is buffered in standard input  and the moment you call another input - it will return immediately because it gets its input from buffer

User · Answer

import sys userinput   sys stdin readline   betAmount   int userinput   print betAmount   This works on my system  I checked int  23 n    would result in 23

User · Answer

Try this      import sys buffer      while True      userinput   sys stdin readline   rstrip   n       if userinput     quit           break     else          buffer append userinput

User · Answer

If you need just one character and you don t want to keep things in the buffer  you can simply read a whole line and drop everything that isn t needed   Replace   stdin read 1    with  stdin readline   strip    1    This will read a line  remove spaces and newlines and just keep the first character

User · Answer

Simon s answer and Volcano s together explain what you re doing wrong  and Simon explains how you can fix it by redesigning your interface   But if you really need to read 1 character  and then later read 1 line  you can do that  It s not trivial  and it s different on Windows vs  everything else   There are actually three cases  a Unix tty  a Windows DOS prompt  or a regular file  redirected file pipe  on either platform  And you have to handle them differently   First  to check if stdin is a tty  both Windows and Unix varieties   you just call sys stdin isatty    That part is cross-platform   For the non-tty case  it s easy  It may actually just work  If it doesn t  you can just read from the unbuffered object underneath sys stdin  In Python 3  this just means sys stdin buffer raw read 1  and sys stdin buffer raw readline    However  this will get you encoded bytes  rather than strings  so you will need to call  decode sys stdin decoding  on the results  you can wrap that all up in a function   For the tty case on Windows  however  input will still be line buffered even on the raw buffer  The only way around this is to use the Console I O functions instead of normal file I O  So  instead of stdin read 1   you do msvcrt getwch     For the tty case on Unix  you have to set the terminal to raw mode instead of the usual line-discipline mode  Once you do that  you can use the same sys stdin buffer read 1   etc   and it will just work  If you re willing to do that permanently  until the end of your script   it s easy  with the tty setraw function  If you want to return to line-discipline mode later  you ll need to use the termios module  This looks scary  but if you just stash the results of termios tcgetattr sys stdin fileno    before calling setraw  then do termios tcsetattr sys stdin fileno    TCSAFLUSH  stash   you don t have to learn what all those fiddly bits mean   On both platforms  mixing console I O and raw terminal mode is painful  You definitely can t use the sys stdin buffer if you ve ever done any console raw reading  you can only use sys stdin buffer raw  You could always replace readline by reading character by character until you get a newline    but if the user tries to edit his entry by using backspace  arrows  emacs-style command keys  etc   you re going to get all those as raw keypresses  which you don t want to deal with

[python] sys.stdin.readline() reads without prompt, returning 'nothing in between'

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