How to set a default string for raw_input?

问题:

I'm using python2.7's raw_input to read from stdin.

I want to let the user change a given default string.

Code:

i = raw_input("Please enter name:")

Console:

Please enter name: Jack

The user should be presented with Jack but can change (backspace) it to something else.

The Please enter name: argument would be the prompt for raw_input and that part shouldn't be changeable by the user.

回答1:

You could do:

i = raw_input("Please enter name[Jack]:") or "Jack"

This way, if user just presses return without entering anything, "i" will be assigned "Jack".

回答2:

Python2.7 get raw_input and set a default value:

Put this in a file called a.py:

import readline
def rlinput(prompt, prefill=''):
   readline.set_startup_hook(lambda: readline.insert_text(prefill))
   try:
      return raw_input(prompt)
   finally:
      readline.set_startup_hook()

default_value = "an insecticide"
stuff = rlinput("Caffeine is: ", default_value)
print("final answer: " + stuff)

Run the program, it stops and presents the user with this:

el@defiant ~ $ python2.7 a.py
Caffeine is: an insecticide

The cursor is at the end, user presses backspace until 'an insecticide' is gone, types something else, then presses enter:

el@defiant ~ $ python2.7 a.py
Caffeine is: water soluable

Program finishes like this, final answer gets what the user typed:

el@defiant ~ $ python2.7 a.py 
Caffeine is: water soluable
final answer: water soluable

Equivalent to above, but works in Python3:

import readline    
def rlinput(prompt, prefill=''):
   readline.set_startup_hook(lambda: readline.insert_text(prefill))
   try:
      return input(prompt)
   finally:
      readline.set_startup_hook()

default_value = "an insecticide"
stuff = rlinput("Caffeine is: ", default_value)
print("final answer: " + stuff)

More info on what's going on here:

https://stackoverflow.com/a/2533142/445131

回答3:

In dheerosaur's answer If user press Enter to select default value in reality it wont be saved as python considers it as '' string so Extending a bit on what dheerosaur.

default = "Jack"
user_input = raw_input("Please enter name: %s"%default + chr(8)*4)
if not user_input:
    user_input = default

Fyi .. The ASCII value of backspace is 08

回答4:

On platforms with readline, you can use the method described here: https://stackoverflow.com/a/2533142/1090657

On Windows, you can use the msvcrt module:

from msvcrt import getch, putch

def putstr(str):
    for c in str:
        putch(c)

def input(prompt, default=None):
    putstr(prompt)
    if default is None:
        data = []
    else:
        data = list(default)
        putstr(data)
    while True:
        c = getch()
        if c in '\r\n':
            break
        elif c == '\003': # Ctrl-C
            putstr('\r\n')
            raise KeyboardInterrupt
        elif c == '\b': # Backspace
            if data:
                putstr('\b \b') # Backspace and wipe the character cell
                data.pop()
        elif c in '\0\xe0': # Special keys
            getch()
        else:
            putch(c)
            data.append(c)
    putstr('\r\n')
    return ''.join(data)

Note that arrows keys don't work for the windows version, when it's used, nothing will happen.

回答5:

I only add this because you should write a simple function for reuse. Here is the one I wrote:

def default_input( message, defaultVal ):
    if defaultVal:
        return raw_input( "%s [%s]:" % (message,defaultVal) ) or defaultVal
    else:
        return raw_input( "%s " % (message) )

回答6:

Try this: raw_input("Please enter name: Jack" + chr(8)*4)

The ASCII value of backspace is 08.