[Python-ideas] Default value for input

[Andrew Barnert]

From: Ron Adam <ron3200 at gmail.com>

Sent: Tuesday, April 1, 2014 6:45 PM


> On 04/01/2014 07:58 AM, Steven D'Aprano wrote:


>>  I often have the need to include a default or initial value when
>>  asking the user for input. In other words, I'd like to call this:
>> 
>>  input("What is the fish of the day? ", "trout a la 
> creme")
>> 
>> 
>>  and have the prompt be "What is the fish of the day? " and the 
> initial
>>  value in the edit buffer be "trout a la creme", fully editable 
> with
>>  whatever line editing tools are available on the user's system. There
>>  are work-arounds for this lack, but they don't make a nice clean UI.
>> 
>>  Under Linux, with readline, I can implement this relatively simply:


> One of the things I first looked for when learning python was a 
> get_key_press function.
> 
> Line input (or whole file input) is great for files, and can make reading 
> data much faster than single character input.   But for user input, single 
> char input can be useful.
> 
> If we could read the keyboard directly with a get_key_press function,  then 
> your request along with making curses cross platform, may be fairly easy to do.
> 
> (The cross platform get_key_press function is the hard part.)


Are you serious?

A get_key_press function is the _easy_ part. On Unix, you use termios to put stdin into raw mode then select or blocking-read stdin; on Windows, you use the msvcrt module to call the wrappers around the console I/O functions.

But what do you think those functions return for, say, left arrow, or ctrl+end? (Even beyond non-character keys, even the characters don't necessarily show up as a single character, but as 1 or more bytes in the appropriate encoding (Unix) or 1 or more UTF-16 codes (Windows), so your state is less trivial than you're expecting…)

And what are you going to do with a left-arrow key press when you get it? If your answer is "move the cursor left", how do you do that? On Windows you use console I/O functions; on Unix, you talk to termcap to figure out which control sequence to send and then send that; etc.

And, once you've moved the cursor left, you probably want to insert rather than overwriting. And of course you need to be able to deal with input that wraps onto multiple lines; you may or may not have to do that wrapping yourself. And so on.

And once you've solved all of that, and figured out how to keep track of buffers and cursor positions, then you just need to write code to emulate everything that each platform does for terminal line editing. To get an idea of how much work that is for each platform, look at the source to readline or libedit.

This is exactly why Python doesn't do any of that; it just uses readline (or libedit's readline-compat mode) on Unix, lets cmd.exe do its thing on Windows, etc. (This also makes it relatively easy for IDEs to hook the behavior and, e.g., make input give you a GUI window with a Tk text entry box.)