How to bind several key presses together in turtle graphics?

Question

I'm trying to make a connect-the-dot python game. I want the game to register 2 button presses. Example: if the user presses Up and Right arrow key, the turtle goes 45 degrees north east.

here is my code:

import turtle

flynn=turtle.Turtle()
win=turtle.Screen()
win.bgcolor("LightBlue")
flynn.pensize(7)
flynn.pencolor("lightBlue")

win.listen()

def Up():
    flynn.setheading(90)
    flynn.forward(25)

def Down():
    flynn.setheading(270)
    flynn.forward(20)

def Left():
    flynn.setheading(180)
    flynn.forward(20)

def Right():
    flynn.setheading(0)
    flynn.forward(20)

def upright():
    flynn.setheading(45)
    flynn.forward(20)

win.onkey(Up, "Up")
win.onkey(Down,"Down")
win.onkey(Left,"Left")
win.onkey(Right,"Right")

Answer 1

I'm skeptical that you can cleanly solve this coordinating variables between onkeypress() and onkeyrelease() events. (Though I'd be pleased to be shown otherwise.) I offer an alternate approach where key presses simply post move requests and a timer applies those requests, whether individual or doubled up:

from turtle import Turtle, Screen

win = Screen()

flynn = Turtle('turtle')

def process_events():
    events = tuple(sorted(key_events))

    if events and events in key_event_handlers:
        (key_event_handlers[events])()

    key_events.clear()

    win.ontimer(process_events, 200)

def Up():
    key_events.add('UP')

def Down():
    key_events.add('DOWN')

def Left():
    key_events.add('LEFT')

def Right():
    key_events.add('RIGHT')

def move_up():
    flynn.setheading(90)
    flynn.forward(25)

def move_down():
    flynn.setheading(270)
    flynn.forward(20)

def move_left():
    flynn.setheading(180)
    flynn.forward(20)

def move_right():
    flynn.setheading(0)
    flynn.forward(20)

def move_up_right():
    flynn.setheading(45)
    flynn.forward(20)

def move_down_right():
    flynn.setheading(-45)
    flynn.forward(20)

def move_up_left():
    flynn.setheading(135)
    flynn.forward(20)

def move_down_left():
    flynn.setheading(225)
    flynn.forward(20)

key_event_handlers = { \
    ('UP',): move_up, \
    ('DOWN',): move_down, \
    ('LEFT',): move_left, \
    ('RIGHT',): move_right, \
    ('RIGHT', 'UP'): move_up_right, \
    ('DOWN', 'RIGHT'): move_down_right, \
    ('LEFT', 'UP'): move_up_left, \
    ('DOWN', 'LEFT'): move_down_left, \
}

key_events = set()

win.onkey(Up, "Up")
win.onkey(Down, "Down")
win.onkey(Left, "Left")
win.onkey(Right, "Right")

win.listen()

process_events()

win.mainloop()

This might take some fine tuning depending on your particular needs. (E.g. how you handle more than two events in key_events).

Answer 2

cdlane has an awesome idea here of using ontimer and a set of currently-pressed keys, but I thought I'd try to extend and refine it a bit.

The problem with a secondary loop with ontimer is that it seems to fight the main turtle loop, both in terms of computation and also in terms of thread safety/interleaving, where you can begin iterating the key pressed set and find that a handler has pulled a key out during iteration, raising an error.

The (seemingly poorly-named) tracer(0) function lets you disable turtle's loop so you can call it manually from within the hand-rolled ontimer loop using update(). This reduces some of the choppiness of the competing loops, although I imagine the timer resolution on rolling your own loop with repeated calls to ontimer is less precise than the built-in loop. But I haven't really looked at the source yet -- feel free to leave a comment if you have any insight.

Here's the proof of concept:

import turtle

def tick():
    for action in keys_pressed:
        actions[action]()

    turtle.update()
    win.ontimer(tick, frame_delay_ms)

t = turtle.Turtle()
turtle.tracer(0)
frame_delay_ms = 1000 // 30 # default for turtle is 10 in _CFG["delay"]
step_speed = 10
actions = dict(
    l=lambda: t.left(step_speed),
    r=lambda: t.right(step_speed),
    u=lambda: t.forward(step_speed),
)
win = turtle.Screen()
keys_pressed = set()
win.onkeypress(lambda: keys_pressed.add("u"), "Up")
win.onkeypress(lambda: keys_pressed.add("l"), "Left")
win.onkeypress(lambda: keys_pressed.add("r"), "Right")
win.onkeyrelease(lambda: keys_pressed.remove("u"), "Up")
win.onkeyrelease(lambda: keys_pressed.remove("l"), "Left")
win.onkeyrelease(lambda: keys_pressed.remove("r"), "Right")
win.listen()
tick()
win.exitonclick()

Ultimately, though, if you want to go much further into realtime graphics and games, Pygame is better equipped.

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cdlane has a few good posts on tracer: 1, 2, 3.