Attributes initialized as None

[dylwil3]

This issue is a place to discuss the convention for attributes initialized as None specifically in the situation where:

• These attributes are not specified using arguments passed in __init__
• These attributes cannot actually be used as type None and have to be assigned.
• The attribute is never used in the form if self.attribute is None: ...

For concreteness, let's focus on a specific example of this. In the current Petting Zoo implementation we have:

class Entity:  # properties and state of physical world entity
    def __init__(self):
        ...
        self.max_speed = None
        ...

and then, in Simple Tag:

def make_world(self, num_good=1, num_adversaries=3, num_obstacles=2):
        ...
        world.agents = [Agent() for i in range(num_agents)]
        for i, agent in enumerate(world.agents):
            agent.adversary = True if i < num_adversaries else False
            ...
            agent.max_speed = 1.0 if agent.adversary else 1.3
            ...

Possible Proposals

1. Do nothing (requires # pyright:ignore because type of attribute is inferred as None.).
2. In the definition of Entity change to:

class Entity:  # properties and state of physical world entity
    def __init__(self):
        ...
        self.max_speed : bool | None = None
        ...

3. In the definition of Entity change to:

class Entity:  # properties and state of physical world entity
    def __init__(self):
        ...
    @property
    def max_speed(self):
        if self._max_speed is None:
            raise NotImplementedError
        return self._max_speed
    @max_speed.setter
    def max_speed(self, val : float):
        self._max_speed = val

4. Something else.

In the most recent PR (#1 ), I took option 3, but I don't have an especially strong stance.

Would love to hear any thoughts folks might have on this!