Problems with user input

[Florian Wilhelm]

I have some easy questions, but I cant figure them out:

I want to read exactly one char from stdin, there should
be no need to hit enter.

import sys

print "Input: (y/N) ",
input = sys.stdin.read( 1)
print "Your input:", input

This code has following problems:
1) I need to hit enter, although I read only one char?!?
2) The complete output is:

Input: (y/N) y
Your input: y

Why does the whitespace char appear in front of "Your input..."?
I guess it has something to do with not causing a linebreak
after "Input: (y/N) ", but I want no linebreak...

Any suggestions?

Regards,
Florian

[F. Petitjean]

Re: Problems with user input

On Tue, 09 Nov 2004 23:54:37 Florian Wilhelm <Florian.Wilhel m@web.de> wrote:[color=blue]
> I have some easy questions, but I cant figure them out:
>
> I want to read exactly one char from stdin, there should
> be no need to hit enter.
>
> import sys
>
> print "Input: (y/N) ",
> input = sys.stdin.read( 1)
> print "Your input:", input
>
> This code has following problems:
> 1) I need to hit enter, although I read only one char?!?[/color]
sys.stdin is : buffered and line oriented.
What is your operating system ? Under unix sys.stdin (and C stdin
stream) are managed by a driver which expect you can somewhat modify
the line you want to send to the system. (POSIX normalized)
If you want a fine control, you can use curses (Unix) or build your
solution based on getchar (or is it getch() ?) on windows/dos.[color=blue]
> 2) The complete output is:
>
> Input: (y/N) y
> Your input: y
>
> Why does the whitespace char appear in front of "Your input..."?[/color]
??[color=blue]
> I guess it has something to do with not causing a linebreak
> after "Input: (y/N) ", but I want no linebreak...
>
> Any suggestions?[/color]
Tell us more about what you want to do.[color=blue]
>
> Regards,
> Florian
>[/color]

[Diez B. Roggisch]

Re: Problems with user input

> I want to read exactly one char from stdin, there should[color=blue]
> be no need to hit enter.[/color]

That's because your terminal is in canonical mode - that means that it only
reads whole lines. You can put it to non-canonical mode - one way is
described here:


http://www.ibiblio.org/obp/py4fun/lode/lode.html#auto2

Please make sure you set it back to canonical mode, or you will expirience
some strange effects.
[color=blue]
> Why does the whitespace char appear in front of "Your input..."?
> I guess it has something to do with not causing a linebreak
> after "Input: (y/N) ", but I want no linebreak...[/color]

The print statement concats an argument list by spaces. Use this instead:

sys.stdout.writ e("Your input:%s\n" % input)

--
Regards,

Diez B. Roggisch

[Florian Wilhelm]

Re: Problems with user input

My OS is Linux 2.6.8.1 and Python version 2.3.4

I just want to write a function that, if called,
asks a yes/no question. The user should only need
to type one char, without confirming with enter.

I wrote a function similar to following

import sys

print "Input: (y/N) ",
input = sys.stdin.read( 1)
print "Your input:", input

If I execute these lines and type "y" followed by "enter".
I get this output

Input: (y/N) y
Your input: y

The problem is that there is a whitespace in front of the
second line, that is not generated by the second print
statement. I dont understand why it appears...

It really has nothing to do with the second "print",
because following code:

import sys

print "Input: (y/N) ",
input = sys.stdin.read( 1)
print "Hello World"
print "Your input:", input

produces this output...

Input: (y/N) y
Hello World
Your input: y

Can somebody explain why the whitespace char appears
in front of "Hello World"???

[Steven Bethard]

Re: Problems with user input

Florian Wilhelm <Florian.Wilhel m <at> web.de> writes:[color=blue]
>
> import sys
>
> print "Input: (y/N) ",
> input = sys.stdin.read( 1)
> print "Hello World"
> print "Your input:", input
>
> produces this output...
>
> Input: (y/N) y
> Hello World
> Your input: y
>
> Can somebody explain why the whitespace char appears
> in front of "Hello World"???[/color]

This is the behavior of the print statement. For example:
[color=blue][color=green][color=darkred]
>>> print "X", sys.stdin.read( 1), "Y"[/color][/color][/color]
X1
1 Y

Every comma between two expressions in the print statement adds a space. As you
can see from the code above, the space isn't inserted until after the following
expression (sys.stdin.read (1)) is executed, and hence I get the space from the
print statement following the newline I inserted when I pressed <ENTER> after
typing 1.

If instead, you use sys.stdout.writ e, you get:
[color=blue][color=green][color=darkred]
>>> def f():[/color][/color][/color]
.... sys.stdout.writ e("X%s" % sys.stdin.read( 1))
.... sys.stdout.writ e("Y\n")
....[color=blue][color=green][color=darkred]
>>> f()[/color][/color][/color]
1
X1Y

sys.stdout.writ e doesn't insert spaces (unless you put spaces in the strings).
In general, if you want a particular format, you should use sys.stdout.writ e
instead of print, since the print statement has a number of such idiosyncracies.

Steve

[Florian Wilhelm]

Re: Problems with user input

Thank you very much!

That solved both problems!

[Terry Reedy]

Re: Problems with user input


"Florian Wilhelm" <Florian.Wilhel m@web.de> wrote in message
news:pan.2004.1 1.09.22.54.37.1 20023@web.de...[color=blue]
> I want to read exactly one char from stdin, there should
> be no need to hit enter.[/color]

Your problem is that terminal input, as seen from C, is usually, by
default, in buffered line mode. This allows the user to edit entries, at
least by back-spacing, before hitting enter. How to switch to character
mode or grab the raw keyboard input is OS specific.
[color=blue]
> Input: (y/N) y
> Your input: y
>
> Why does the whitespace char appear in front of "Your input..."?[/color]

Because the print statement puts spaces between items so you don't have to.
If the convenience of print does not do what you want, take control by
using sys.stdout.writ e(<string>), which writes *exactly* what you tell it
to.

Terry J. Reedy