Detect arrow keys being pressed on console in Python - linux

Question

I am trying to write a console program in Python 3 that provides some sort of shell for the user, just like the Python 3 shell in a console. I was able to achieve this relatively quickly by using the input()method. However, it would be nice if, in that shell, one could use the arrow keys to cycle through the most recently typed commands, just like you can in other shells. The input() method does not provide this feature, and I did not find any other simple tools to do this, except for the curses module, which needs to take over the whole screen to work. One of my approaches was to read the typed text from stdin byte by byte and then check it against the codes for the special characters I'm looking for. This works pretty well, but it would run into problems when the user (for some reason) types a weird unicode character that contains the code for a key like the arrow key somewhere in the middle. While this is still an acceptable solution for me, I feel like this is a problem which ought to have been solved (better) before, given how often it has got to occur.

Answer 1

In Python 3, sys.stdin.read returns unicode characters as a single character. Escape sequences for arrow keys are delivered as multiple ASCII characters. Here is an example program , using tty and termios, which parses inputs accordingly.
import sys,tty,termios
# Commands and escape codes
END_OF_TEXT = chr(3) # CTRL+C (prints nothing)
END_OF_FILE = chr(4) # CTRL+D (prints nothing)
CANCEL = chr(24) # CTRL+X
ESCAPE = chr(27) # Escape
CONTROL = ESCAPE +'['
# Escape sequences for terminal keyboard navigation
ARROW_UP = CONTROL+'A'
ARROW_DOWN = CONTROL+'B'
ARROW_RIGHT = CONTROL+'C'
ARROW_LEFT = CONTROL+'D'
KEY_END = CONTROL+'F'
KEY_HOME = CONTROL+'H'
PAGE_UP = CONTROL+'5~'
PAGE_DOWN = CONTROL+'6~'
# Escape sequences to match
commands = {
ARROW_UP :'up arrow',
ARROW_DOWN :'down arrow',
ARROW_RIGHT:'right arrow',
ARROW_LEFT :'left arrow',
KEY_END :'end',
KEY_HOME :'home',
PAGE_UP :'page up',
PAGE_DOWN :'page down',
}
# Blocking read of one input character, detecting appropriate interrupts
def getch():
k = sys.stdin.read(1)[0]
if k in {END_OF_TEXT, END_OF_FILE, CANCEL}: raise KeyboardInterrupt
print('raw input 0x%X'%ord(k),end='\r\n')
return k
# Println for raw terminal mode
def println(*args):
print(*args,end='\r\n',flush=True)
# Preserve current terminal settings (we will restore these before exiting)
fd = sys.stdin.fileno()
old_settings = termios.tcgetattr(fd)
try:
# Enter raw mode (key events sent directly as characters)
tty.setraw(sys.stdin.fileno())
# Loop, waiting for keyboard input
while 1:
# Parse known command escape sequences
read = getch()
while any(k.startswith(read) for k in commands.keys()):
if read in commands:
println('detected command (%s)'%commands[read])
read = ''
break
read += getch()
# Interpret all other inputs as text input
for c in read:
println('detected character 0x%X %c'%(ord(c),c))
# Always clean up
finally:
termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
println('')
sys.exit(0)