How to access the joysticks of a gamepad using python evdev?

Question

I'm very new to evdev and am currently trying to use an xbox controller to control a small robot. I am successful with each of the buttons, but I can not seem to figure out how to get valid readings from the joysticks.

This code only works for button response.

edit: Whenever an ABXY button is pressed, EV_ABS events occur also. I am not sure how to distinguish between these values and actual values of the joystick. Additionally, the event codes from the joystick movements are inconsistent. Here is the code I'm currently running:

from evdev import InputDevice, categorize, ecodes
dev = InputDevice('/dev/input/event1')
print(dev)
for event in dev.read_loop():
 if event.type == ecodes.EV_KEY:
 print("button is pressed")
 elif event.type == ecodes.EV_ABS:
 print(event)

Any advice is appreciated! Thanks!

• rodrigo: You'll get EV_ABS and/or EV_REL events for the joysticks. The exact details depend on the gamepad, but I don't have an xbox around :-(
• s.t: Thank you so much for the quick reply! I tried that, but when pressing and releasing buttons, events with EV_ABS values also occur. I'm not quite sure why. The event codes for the joysticks are also very inconsistent.
• rodrigo: Ah, but that is because in some gamepads, some buttons are arranged as a cross (I think it is called d-pad). And those are sometimes mapped by the driver to a hat, that in Linux is a pair of EV_ABS axes. Those ABS_HAT* axes usually only have three possible values: -MAX, 0 and +MAX, that represent one button, no button and the other button, respectively, just as those old arcade digital joysticks.
• rodrigo: But note that I said some gamepads and sometimes... For example, my chinese cheapo gamepad has a Mode button that selects between analog joystick plus a buch of buttons and digital joystick plus a hat and fewer buttons...
• s.t: The buttons that are returning multiple events are the ABXY buttons. How do I distinguish between these events and the events from the actual joysticks?

Answer 1

Python is not my favorite language, and there is probably a better method of calibrating, but I slapped this together. Hope it helps.

All the mappings and maximums were measured from my Xbox One Wireless controller, so you may need to experiment/adjust the constants to your own controller.

from evdev import list_devices, InputDevice, categorize, ecodes
CENTER_TOLERANCE = 350
STICK_MAX = 65536
dev = InputDevice( list_devices()[0] )
axis = {
 ecodes.ABS_X: 'ls_x', # 0 - 65,536 the middle is 32768
 ecodes.ABS_Y: 'ls_y',
 ecodes.ABS_Z: 'rs_x',
 ecodes.ABS_RZ: 'rs_y',
 ecodes.ABS_BRAKE: 'lt', # 0 - 1023
 ecodes.ABS_GAS: 'rt',
 ecodes.ABS_HAT0X: 'dpad_x', # -1 - 1
 ecodes.ABS_HAT0Y: 'dpad_y'
}
center = {
 'ls_x': STICK_MAX/2,
 'ls_y': STICK_MAX/2,
 'rs_x': STICK_MAX/2,
 'rs_y': STICK_MAX/2
}
last = {
 'ls_x': STICK_MAX/2,
 'ls_y': STICK_MAX/2,
 'rs_x': STICK_MAX/2,
 'rs_y': STICK_MAX/2
}
for event in dev.read_loop():
 # calibrate zero on Y button
 if event.type == ecodes.EV_KEY:
 if categorize(event).keycode[0] == "BTN_WEST":
 center['ls_x'] = last['ls_x']
 center['ls_y'] = last['ls_y']
 center['rs_x'] = last['rs_x']
 center['rs_y'] = last['rs_y']
 print( 'calibrated' )
 #read stick axis movement
 elif event.type == ecodes.EV_ABS:
 if axis[ event.code ] in [ 'ls_x', 'ls_y', 'rs_x', 'rs_y' ]:
 last[ axis[ event.code ] ] = event.value
 value = event.value - center[ axis[ event.code ] ]
 if abs( value ) <= CENTER_TOLERANCE:
 value = 0
 if axis[ event.code ] == 'rs_x':
 if value < 0:
 print('left')
 else:
 print('right')
 print( value )
 elif axis[ event.code ] == 'ls_y':
 if value < 0:
 print('foreward')
 else:
 print('backward')
 print( value )