Newb Newb - 2 months ago 15
Python Question

Finding the Values of the Arrow Keys in Python: Why are they triples?

I am trying to find the values that my local system assigns to the arrow keys, specifically in Python. I am using the following script to do this:

import sys,tty,termios
class _Getch:
def __call__(self):
fd = sys.stdin.fileno()
old_settings = termios.tcgetattr(fd)
try:
tty.setraw(sys.stdin.fileno())
ch = sys.stdin.read(1)
finally:
termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
return ch

def get():
inkey = _Getch()
while(1):
k=inkey()
if k!='':break
print 'you pressed', ord(k)

def main():
for i in range(0,25):
get()

if __name__=='__main__':
main()


Then I ran the script, and hit UP DOWN RIGHT LEFT, which gave me this output:

$ python getchar.py
you pressed 27
you pressed 91
you pressed 65
you pressed 27
you pressed 91
you pressed 66
you pressed 27
you pressed 91
you pressed 67
you pressed 27
you pressed 91
you pressed 68


This is anomalous because it suggests that the arrow keys are registered as some form of triple (27-91-6x) on my system, as each press of an arrow key takes up three instances of get(). By comparison, pressing a,b,c and CTRL-C gives:

you pressed 97
you pressed 98
you pressed 99
you pressed 3


Can anyone explain to me why the values of my arrow-keys seem to be stored as triples? Why is this is so? Is this the same across all platforms? (I'm using Debian Linux.) If not, how should I go about storing the values of the arrow-keys?

The end goal here is in that I'm trying to write a program which needs to correctly recognize arrow-keys and perform a function depending on which arrow-key was pressed.

Answer

I think I figured it out.

I learned from here that each arrow key is represented by a unique ANSI escape code. Then I learned that the ANSI escape codes vary by system and application: in my terminal, hitting cat and pressing the up-arrow gives ^[[A, in C it seems to be \033[A, etc. The latter part, the [A, remains the same, but the code for the preceding Escape can be in hex(beginning with an x), octal (beginning with a 0), or decimal(no lead in number).

Then I opened the python console, and plugged in the triples I had previously received, trying to find their character values. As it turned out, chr(27) gave \x1b, chr(91) gave [, and calling chr on 65,66,67,68 returned A,B,C,D respectively. Then it was clear: \x1b was the escape-code!

Then I noted that an arrow key, in ANSI represented as a triple, is of course represented as three characters, so I needed to amend my code so as to read in three characters at a time. Here is the result:

import sys,tty,termios
class _Getch:
    def __call__(self):
            fd = sys.stdin.fileno()
            old_settings = termios.tcgetattr(fd)
            try:
                tty.setraw(sys.stdin.fileno())
                ch = sys.stdin.read(3)
            finally:
                termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
            return ch

def get():
        inkey = _Getch()
        while(1):
                k=inkey()
                if k!='':break
        if k=='\x1b[A':
                print "up"
        elif k=='\x1b[B':
                print "down"
        elif k=='\x1b[C':
                print "right"
        elif k=='\x1b[D':
                print "left"
        else:
                print "not an arrow key!"

def main():
        for i in range(0,20):
                get()

if __name__=='__main__':
        main()
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