Why does Python try-except cause slowdown?

Question

I have written a simple application for my RaspberryPi in python3. My rpi has a fresh install and I have completed all updates prior to starting development. This application is a sports scoreboard that displays on an LED matrix and utilizes a bluetooth game controller and the evdev module for input controls.

The bluetooth input was working very well, and was 100% responsive except that if it disconnected, the application would crash because it could no longer access /dev/input/event0 (which was is the gamepad input).

I added try-except Exception to the code, which resolved the crash and allowed the code to continue if the gamepad disconnected. This has introduced two new issues that I need assistance with.

About 10% of gamepad input doesn't register now. Sometimes a button has to be pressed twice to register.
Ctl-C in the console will not stop code execution. I'll get the following in the console:

KeyboardInterrupt: ^CException ignored in: <function InputDevice.del at 0x757d6588>

If I remove the try-except for the gamepad read, it will work normally until the gamepad disconnects. Here is the code in question.

while True:
    #Gamepad read
    try:
        dev = InputDevice('/dev/input/event0')    
        gen = dev.read()
    except Exception:
        pass
    #Gamepad functions
    try:
        for ev in gen:
            if ev.type == ecodes.EV_KEY:
                if ev.value == 1:
                    if ev.code == 23:
                        if home == 0:
                            pass
                        else:
                            home-=1 
                    elif ev.code == 35:
                        home+=1 
                    elif ev.code == 34:
                        away+=1 
                    elif ev.code == 36:
                        if away==0:
                            pass
                        else:
                            away-=1
                    elif ev.code == 49:
                        stopwatch.reset()
                        start_clock == 300
                    elif ev.code == 32:
                        start_clock+=1
                    elif ev.code == 46:
                        start_clock-=1
                    elif ev.code == 33:
                        start_clock+=60
                    elif ev.code == 18:
                        start_clock-=60        
                    elif ev.code == 24:
                        away=00
                        home=00
                    elif ev.code == 37:
                        _draw_ani()
                    elif ev.code == 50:
                        if stopwatch.running:
                            stopwatch.stop()
                        else:
                            stopwatch.start()   
    except IOError:
        pass

Why are you creating the InputDevice object in every iteration? — AlanWik, Jan 09 '21 at 21:09
`except Exception` is a bad idea, catch the specific error instead. — SuperStormer, Jan 09 '21 at 21:38

score 1 · Answer 1 · answered Jan 09 '21 at 21:34

Generally checking with try/except is costly specially if the exception is raised.This question is a good reference and can help you in that matter.

I think using if/else condition will reduce the load.To do that you should first detect why exception is raised (you can use except Exception as error and the analyze the error object). Then you can check the condition with if/else.

And regarding your code use with instead of opening and reading directly like this:

with InputDevice('/dev/input/event0').read() as gen:
    # do stuff with gen obj

This way you wouldn't need to close obj manually. Right now one of the problems in your code is trying to read a device while it's open.

Why does Python try-except cause slowdown?

1 Answers1