Reading keypresses in Python (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Sat, 25 Mar 2017

Reading keypresses in Python

As part of preparation for Everyone Does IT, I was working on a silly hack to my Python script that plays notes and chords: I wanted to use the computer keyboard like a music keyboard, and play different notes when I press different keys. Obviously, in a case like that I don't want line buffering -- I want the program to play notes as soon as I press a key, not wait until I hit Enter and then play the whole line at once. In Unix that's called "cbreak mode".

There are a few ways to do this in Python. The most straightforward way is to use the curses library, which is designed for console based user interfaces and games. But importing curses is overkill just to do key reading.

Years ago, I found a guide on the official Python Library and Extension FAQ: Python: How do I get a single keypress at a time?. I'd even used it once, for a one-off Raspberry Pi project that I didn't end up using much. I hadn't done much testing of it at the time, but trying it now, I found a big problem: it doesn't block.

Blocking is whether the read() waits for input or returns immediately. If I read a character with c = sys.stdin.read(1) but there's been no character typed yet, a non-blocking read will throw an IOError exception, while a blocking read will wait, not returning until the user types a character.

In the code on that Python FAQ page, blocking looks like it should be optional. This line:

fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)
is the part that requests non-blocking reads. Skipping that should let me read characters one at a time, block until each character is typed. But in practice, it doesn't work. If I omit the O_NONBLOCK flag, reads never return, not even if I hit Enter; if I set O_NONBLOCK, the read immediately raises an IOError. So I have to call read() over and over, spinning the CPU at 100% while I wait for the user to type something.

The way this is supposed to work is documented in the termios man page. Part of what tcgetattr returns is something called the cc structure, which includes two members called Vmin and Vtime. man termios is very clear on how they're supposed to work: for blocking, single character reads, you set Vmin to 1 (that's the number of characters you want it to batch up before returning), and Vtime to 0 (return immediately after getting that one character). But setting them in Python with tcsetattr doesn't make any difference.

(Python also has a module called tty that's supposed to simplify this stuff, and you should be able to call tty.setcbreak(fd). But that didn't work any better than termios: I suspect it just calls termios under the hood.)

But after a few hours of fiddling and googling, I realized that even if Python's termios can't block, there are other ways of blocking on input. The select system call lets you wait on any file descriptor until has input. So I should be able to set stdin to be non-blocking, then do my own blocking by waiting for it with select.

And that worked. Here's a minimal example:

import sys, os
import termios, fcntl
import select

fd = sys.stdin.fileno()
newattr = termios.tcgetattr(fd)
newattr[3] = newattr[3] & ~termios.ICANON
newattr[3] = newattr[3] & ~termios.ECHO
termios.tcsetattr(fd, termios.TCSANOW, newattr)

oldterm = termios.tcgetattr(fd)
oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)

print "Type some stuff"
while True:
    inp, outp, err = select.select([sys.stdin], [], [])
    c = sys.stdin.read()
    if c == 'q':
        break
    print "-", c

# Reset the terminal:
termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)

A less minimal example: keyreader.py, a class to read characters, with blocking and echo optional. It also cleans up after itself on exit, though most of the time that seems to happen automatically when I exit the Python script.

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[ 12:42 Mar 25, 2017    More programming | permalink to this entry | comments ]
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