Interface.py

snake = t.zeros((32, 32), dtype=t.int)
snake[0, :3] = T([1, 2, -1])

fig, ax = plt.subplots(1, 1)
img = ax.imshow(snake)
action = {'val': 1}
action_dict = {'a': 0, 'd': 2}

fig.canvas.mpl_connect('key_press_event', lambda e:
                       action.__setitem__('val', action_dict[e.key]))

score = None
while score is None:
    img.set_data(snake)
    fig.canvas.draw_idle()
    plt.pause(0.1)
    score = do(snake, action['val'])
    action['val'] = 1

print('Score:', score)

This line doesn't change the default value of the dictionary, it sets action_dict[1] to None.

action_dict.setdefault(1)

May I suggest:

LEFT, STRAIGHT, RIGHT = 0, 1, 2
keys = {'a': LEFT, 'd': RIGHT}
action = STRAIGHT
def on_key(e):
  global action
  action = keys.get(e.key, STRAIGHT)

fig.canvas.mpl_connect('key_press_event', on_key)

score = None
while score is None:
    img.set_data(snake)
    fig.canvas.draw_idle()
    plt.pause(0.1)
    score = do(snake, action)
    action = STRAIGHT