sdl2实体的错误设置属性

Question

I am using the pysdl2 library. self.velocity will print, but self.forward_tick with throw a key error.

What is causing only part of the self attributes from being assigned. I am thinking it has something to do with the inheritance.

class Robot(sdl2.ext.Entity):
    def __init__(self, world, sprite, posx=0, posy=0):
        self.sprite = sprite
        self.sprite.position = posx, posy
        self.velocity = Velocity()
        self.forward_tick = 0
        self.go_forward = 0
        self.unit_forward = (1,0)
        print(self.velocity)
        print(self.forward_tick)

Here is the output:

Collins-MacBook-Air:soccer_bots collinbell$ python test_simulation_world.py 
<simulation_world.Velocity object at 0x108ecb5c0>
Traceback (most recent call last):
  File "/Users/collinbell/.pyenv/versions/3.4.3/lib/python3.4/site-packages/sdl2/ext/ebs.py", line 53, in __getattr__
    ctype = self._world._componenttypes[name]
KeyError: 'forward_tick'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test_simulation_world.py", line 3, in <module>
    world = Simulation_World("Test Simulation", 1080, 720)
  File "/Users/collinbell/Programs/soccer_bots/simulation_world.py", line 100, in __init__
    self.player1 = Robot(self.world, sp_paddle1, 0, 250)
  File "/Users/collinbell/Programs/soccer_bots/simulation_world.py", line 22, in __init__
    print(self.forward_tick)
  File "/Users/collinbell/.pyenv/versions/3.4.3/lib/python3.4/site-packages/sdl2/ext/ebs.py", line 56, in __getattr__
    (self.__class__.__name__, name))
AttributeError: object ''Robot'' has no attribute ''forward_tick''

Answer 1

From the docs on component-based design with sdl2.ext , the Entity type is special, and doesn't follow normal Python idioms. In particular, you can't just create arbitrary attributes; you can only create attributes whose value is a component type, and whose name is a lowercased version of that type:

Entity objects define the in-application objects and only consist of component-based attributes .

…

The Entity also requries its attributes to be named exactly as their component class name, but in lowercase letters.

So, when you try to add an attribute named forward_tick , that causes Entity.__setattr__ to go looking for a class named Forward_Tick in the world's component types. Which it apparently does by looking up self._world._componentypes[name] , which is the line that's actually raising the exception, as you can see in the tracebacks.

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Without knowing anything more about your code and your design than the tiny fragment you showed us, I can't tell you how to fix this. But most likely, it's one of the following:

1. You actually wanted to create a Component , not an Entity ,
2. You wanted to wrap up forward_tick in a Component type that this Entity can contain, or
3. You didn't want a Component -oriented design in the first place.

At any rate, as the docs say:

If you are just starting with such a [component-oriented] design, it is recommended to read through the The Pong Game tutorial .