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capture key press

Posted on 2010-11-11
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Last Modified: 2012-05-10
Using Python3 (console mode).

I am trying to capture key press (restrict to certain digits and charaters) - non-allows characters presses not displayed otherwise see a text string appear on screen.

New to Python -- advise much appreciated
import msvcrt        



def doKeyEvent(key):

    if key == '\x00' or key == '\xe0':    # non ASCII

       key = msvcrt.getch()

    print (ord(key),)



def doQuitEvent(key):

    raise SystemExit





def keyTest1() :





    ky = msvcrt.getch()

    length = len(ky)

    if length != 0:         

        if ky == " ":

            doQuitEvent(ky)

        else: 

            doKeyEvent(ky)

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Question by:amillyard
  • 6
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9 Comments
 
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Expert Comment

by:pepr
ID: 34118512
Try the following snippet to see, what is returned.  Now, the important thing is to understand that there is a strict difference between strings and bytes in Python3.  The msvcrt.getch() returns the bytes type (literals having b in front of the first quotation mark).  This way, you have to convert the bytes explicitly to get the string.  Strings in Python3 are always in Unicode.  This also means that you have to tell the encoding for the conversion.

I do recommend to read the Strings chapter (http://diveintopython3.org/strings.html) from Dive Into Python 3 by Mark Pilgrim (http://diveintopython3.org/), namely the section Strings vs. Bytes (http://diveintopython3.org/strings.html#byte-arrays).
import msvcrt        

k = None          # init
while k != b' ':  # press space to leave the loop
    k = msvcrt.getch()
    print(repr(k))

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Author Comment

by:amillyard
ID: 34119051
thanks for the feedback.

when I apply the above scripting... the   b'\xff'  constantly streams upwards.

what I am hoping to achieve is text entry on a single line ... any non-allowed character  (from a list eventually) simply does not get added to the input (ignored).

at the moment I cannot break unless using Ctrl-C



b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
b'\xff'
Traceback (most recent call last):
  File "C:\Users\Administrator\Desktop\ass1.py", line 357, in <module>
    elif len(line)==1 and "1"<=line<="9" : exlist[int(line)]()
  File "C:\Users\Administrator\Desktop\ass1.py", line 248, in ex8
    print(repr(k))
KeyboardInterrupt
>>>
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Expert Comment

by:pepr
ID: 34119113
Can you show your code from around the line 248 in ass1.py?
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LVL 28

Expert Comment

by:pepr
ID: 34119114
... and around 357
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Author Comment

by:amillyard
ID: 34119120
there are basically just a bunch of functions ... the break was the ctrl-c to force the stop... otherwise -the scripting loops on itself (the above scripting)
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Expert Comment

by:pepr
ID: 34119156
I am just curious where the b'\xff' came from.
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Author Comment

by:amillyard
ID: 34119168
this is the only scripting in that section - a loop to select different functions to test various routines as attached
# do not modify anything beneath this line

exlist = [None, ex1, ex2, ex3, ex4, ex5, ex6, ex7, ex8, ex9]

running = True

while running :

    line = input("\nSelect option (0 to quit): ")

    if line == "0" : running = False

    elif line == "10" : ex10()

    elif len(line)==1 and "1"<=line<="9" : exlist[int(line)]()

    else : print("Invalid input - try again")

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Expert Comment

by:pepr
ID: 34119627
I am afraid you cannot combine the standard input() function and the msvcrt.getch() together.  I may be wrong but you need to write your own implementation of the input() -- say myInput().  Try the code below to discuss about.  It uses getch() to get the characters of the keys and collects them in the list of single-char strings.  However, it is rather simplistic as it works only with ASCII characters.  Try to press the arrow key or some accented character and you will know what I mean.  On the other hand, there are places in the code where the tests could be added to solve the situations.
import msvcrt        

k = None      # init
lst = []
while k != b'\r':
    k = msvcrt.getch()
    if k != b'\r':
       msvcrt.putch(k)
       lst.append(k.decode('ascii'))

msvcrt.putch(b'\n') 
    
print(repr(''.join(lst)))

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Accepted Solution

by:
pepr earned 500 total points
ID: 34119888
Try the following myInput() function.  You may want to implement it better so that it could run also on non-Windows OS.
import msvcrt
import sys


def myInput(prompt):
    sys.stdout.write(prompt + ' ')
    sys.stdout.flush()
    k = None                               # init the key value
    lst = []                               # list of accepted chars
    while k != b'\r':
        k = msvcrt.getch()                 # get the key 
        if k == b'\xe0':                   # arrow key sequence?
            x = msvcrt.getch()             # the arrow identifier
            sys.stdout.write('\a')         # beep
            sys.stdout.flush()
        
        # Filter out the unwanted characters here.
        # Possibly beep if you do not like it -- here, do not allow 'a'.
        elif k == b'a':
            sys.stdout.write('\a')
            sys.stdout.flush()
            
        elif k != b'\r':         
            c = k.decode(sys.stdin.encoding) # to be displayed
            sys.stdout.write(c)
            sys.stdout.flush()
            lst.append(c)

    sys.stdout.write('\n')
    return ''.join(lst)
    
    
print(myInput('my prompt:'))    
print(myInput("try to enter 'a' char:"))    
print(myInput('try to press the arrow key:'))

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