Attributes defined for specific instances only

[dylwil3]

This issue is a place to discuss the conventions for attributes which are defined for a specific instance of a class only.

Here is a concrete example. In the current implementation of the World class, we have:

class World:  # multi-agent world
    def __init__(self):
        # list of agents and entities (can change at execution-time!)
        self.agents = []
        self.landmarks = []
        # communication channel dimensionality
        self.dim_c = 0
        # position dimensionality
        self.dim_p = 2
        # color dimensionality
        self.dim_color = 3
        # simulation timestep
        self.dt = 0.1
        # physical damping
        self.damping = 0.25
        # contact response parameters
        self.contact_force = 1e2
        self.contact_margin = 1e-3

In particular, there is no attribute called food. However, in the code for the Scenario of Simple World Comm, we see:

class Scenario(BaseScenario):
    def make_world(
        self,
        num_good_agents=2,
        num_adversaries=4,
        num_landmarks=1,
        num_food=2,
        num_forests=2,
    ):
        world = World()
        ...
        # add agents
        world.agents = [Agent() for i in range(num_agents)]
        ...
        world.food = [Landmark() for i in range(num_food)]
        ...

Proposals

1. Do nothing. (Requires # pyright:ignore).
2. Change definition of World to include:

...
self.food = []

This still requires # pyright : ignore.
(NB : While it's not the case for this specific example, if we had to do something like self.attribute = None, we would then be in the situation of Issue #2 .)
3. Change definition of World to include:

...
self.food : list[Landmark] = []

4. Something else.

In the current PR #1 , I've chosen option 3. I'm not wedded to this.

Would love to hear people's thoughts!