Environments

sekai.Env[ObsType, ActType] is the base class of every single-agent environment. It keeps gymnasium's shape (reset, step, two spaces) and changes what comes back: structured results instead of tuples, async methods on every env, and no rendering on the env itself.

The interface

Member	Kind	Notes
observation_space	attribute	a sekai.Space; set it in __init__
action_space	attribute	a sekai.Space; set it in __init__
reset(*, seed=None, options=None)	abstract	returns a ResetResult
step(action)	abstract	returns a StepResult
async_reset(...), async_step(action)	coroutine	default to the sync methods; override for real async I/O (Async)
close()	method	release resources; no-op by default
rng	property	the env's numpy.random.Generator
_set_rng(seed)	method	seed rng; call it first in reset(); returns the seed in use
metadata	class attribute	free-form dict, e.g. {"render_fps": 50}
spec	attribute	the EnvSpec used by sekai.make(), else None
unwrapped	property	the base env under any wrappers (itself, for a base env)
has_attr(name), get_attr(name)	method	look an attribute up through the wrapper stack
with env:	context manager	calls close() on exit

There is no render(), render_mode or np_random: rendering is done by Renderer objects and the generator is env.rng.

Results

step() returns a StepResult and reset() a ResetResult. Both are frozen, slotted dataclasses, generic in the observation type.

	StepResult	ResetResult
observation	next observation	first observation
reward	float	—
terminated	reached a terminal state of the MDP	—
truncated	cut short (time limit, data ran out, ...)	—
info	dict[str, Any]	dict[str, Any]
done	terminated or truncated	always False

Because ResetResult.done is always False, a single variable can hold either result:

import sekai

env = sekai.make("CartPole-v1")
env.action_space.seed(0)
result = env.reset(seed=0)
while not result.done:
    result = env.step(env.action_space.sample())
print(type(result).__name__, result.terminated, result.truncated)  # StepResult True False

Results are immutable, which makes them safe to store in replay buffers or send across threads. Use dataclasses.replace() to derive a modified copy:

import dataclasses

try:
    result.reward = 0.0
except dataclasses.FrozenInstanceError as e:
    print(e)  # cannot assign to field 'reward'

zeroed = dataclasses.replace(result, reward=0.0)
print(result.reward, zeroed.reward)  # 1.0 0.0

terminated vs truncated

Set terminated=True when the episode reached a terminal state of the underlying MDP (the pole fell, the inventory is sold, the session closed and the agent can see the clock). Set truncated=True when it was cut off for a reason outside the MDP (a time limit the agent cannot observe, the end of a historical price file). Value-based learners bootstrap from truncated states but not from terminated ones, so the distinction matters.

Seeding

Every env owns one numpy.random.Generator, env.rng. The contract:

Call	Effect
reset(seed=int)	re-seed rng deterministically
reset(seed=None) (or reset())	keep the current generator, so the stream continues
the very first reset() without a seed	create a generator from 128 bits of OS entropy
reset(seed=np.random.Generator)	use that generator directly (the recorded seed is -1)

So reset(seed=s) followed by plain reset() calls gives one reproducible sequence of episodes, exactly as in gymnasium:

first = sekai.make("CartPole-v1")
second = sekai.make("CartPole-v1")

a = [first.reset(seed=7).observation] + [first.reset().observation for _ in range(3)]
b = [second.reset(seed=7).observation] + [second.reset().observation for _ in range(3)]
print(all((x == y).all() for x, y in zip(a, b)))  # True

Inside your environment, call self._set_rng(seed) as the first line of reset() and draw every random number from self.rng. Never use np.random.* module functions or Python's random: they bypass the seed.

Spaces are seeded separately

env.action_space.sample() draws from the space's generator, which env.reset(seed=...) does not touch. Call env.action_space.seed(s) for reproducible random actions (as in gymnasium).

Wrappers have no generator of their own: wrapper.rng is the base env's rng, and wrapper.reset(seed=s) seeds the base env. The one exception is StickyAction, which derives a private generator from the reset seed so that it does not perturb the env's stream.

Writing an environment

A complete environment: an inventory manager that orders stock each day to meet random demand over a season.

import numpy as np
import sekai


class InventoryEnv(sekai.Env[dict, np.intp]):
    """Order stock each day to meet Poisson demand over a season."""

    metadata = {"render_fps": 4}

    def __init__(self, capacity: int = 20, mean_demand: float = 4.0, days: int = 30,
                 price: float = 2.0, unit_cost: float = 1.0, holding_cost: float = 0.1):
        self.capacity, self.mean_demand, self.days = capacity, mean_demand, days
        self.price, self.unit_cost, self.holding_cost = price, unit_cost, holding_cost
        self.observation_space = sekai.Dict({
            "stock": sekai.Box(0, capacity, shape=(1,), dtype=np.int64),
            "days_left": sekai.Discrete(days + 1),
        })
        self.action_space = sekai.Discrete(capacity + 1)  # units to order
        self.stock = None

    def reset(self, *, seed=None, options=None):
        self._set_rng(seed)                       # 1. seed first
        options = options or {}
        self.stock = int(options.get("initial_stock", 0))
        self.day = 0
        return sekai.ResetResult(self._obs(), info={})

    def step(self, action):
        if self.stock is None:
            raise sekai.ResetRequired()           # 2. fail loudly on misuse
        if not self.action_space.contains(action):
            raise sekai.InvalidAction(action, self.action_space)
        ordered = min(int(action), self.capacity - self.stock)
        self.stock += ordered
        demand = int(self.rng.poisson(self.mean_demand))   # 3. randomness from self.rng
        sold = min(demand, self.stock)
        self.stock -= sold
        self.day += 1
        reward = self.price * sold - self.unit_cost * ordered - self.holding_cost * self.stock
        return sekai.StepResult(
            observation=self._obs(),              # 4. a fresh object every step
            reward=float(reward),
            terminated=self.day >= self.days,     # the clock is observed: a true terminal
            truncated=False,
            info={"demand": demand, "sold": sold, "lost_sales": demand - sold},
        )

    def _obs(self):
        return {"stock": np.array([self.stock], dtype=np.int64),
                "days_left": np.intp(self.days - self.day)}

Check it, then run a base-stock policy ("order up to 6 units"):

print(sekai.check_env(InventoryEnv(), check_determinism=True))  # []

env = InventoryEnv()
result = env.reset(seed=0, options={"initial_stock": 5})
total = 0.0
while not result.done:
    result = env.step(max(0, 6 - int(result.observation["stock"][0])))
    total += result.reward
print(round(total, 1), result.info)  # 110.5 {'demand': 3, 'sold': 3, 'lost_sales': 0}

The checklist, which check_env verifies for you:

1. Set observation_space and action_space in __init__.
2. Call self._set_rng(seed) first in reset(); draw all randomness from self.rng.
3. Return observations that lie in observation_space, with the space's dtype.
4. Return a new observation object each step; never mutate one you already returned (a replay buffer may still hold it).
5. Return a float reward and bool flags; keep info a dict.
6. Accept options in reset() for anything configurable per episode.

Type parameters

sekai.Env[ObsType, ActType] is generic, so a type checker knows that env.step(a).observation is, say, an np.ndarray. The parameters are optional; class MyEnv(sekai.Env) works too.

Metadata and spec

metadata is a free-form class-level dict. sekai reads one key: render_fps, used by RecordFrames.save() and passed through by to_gymnasium().

spec is set by sekai.make() to the EnvSpec that built the env, including the keyword arguments actually used, so env.spec.make() builds an identical copy. Environments constructed directly have spec = None.

env = sekai.make("Portfolio-v0", n_assets=3, episode_length=20)
print(env, env.spec.kwargs)  # <Portfolio-v0> {'n_assets': 3, 'episode_length': 20}

clone = env.spec.make()
print(clone.unwrapped.n_assets)  # 3
print(sekai.envs.CartPoleEnv())  # <CartPoleEnv>

Lifecycle

close() releases whatever the env holds (files, sockets, subprocesses). Environments are context managers, so with guarantees it runs:

with sekai.make("CartPole-v1") as env:
    env.reset(seed=0)
    env.step(0)
# env.close() has been called here, even if the block raised

Reaching through wrappers

Wrappers do not forward unknown attributes (gymnasium 1.x dropped that too). Use unwrapped for the base env, or get_attr() / has_attr(), which search the stack from the outside in:

env = sekai.make("CartPole-v1")                 # TimeLimit(CartPoleEnv)
print(env.has_attr("x_threshold"), env.get_attr("x_threshold"))  # True 2.4
print(env.unwrapped.x_threshold)                # 2.4
print(env.get_attr("max_episode_steps"))        # 500

len(wrapper) counts the layers and iterating over a wrapper walks from the outermost layer to the base env; see Wrappers.