Make your own custom environment

This documentation overviews creating new environments and relevant useful wrappers, utilities and tests included in Gymnasium designed for the creation of new environments.


Alternative solutions

Install Copier with Pip or Conda:

pip install copier


conda install -c conda-forge copier

Generate your environment

You can check that Copier has been correctly installed by running the following command, which should output a version number:

copier --version

Then you can just run the following command and replace the string path/to/directory by the path to the directory where you want to create your new project.

copier copy "path/to/directory"

Answer the questions, and when it’s finished you should get a project structure like the following:

├── gymnasium_env
│   ├── envs
│      ├──
│      └──
│   ├──
│   └── wrappers
│       ├──
│       ├──
│       ├──
│       ├──
│       └──
├── pyproject.toml

Subclassing gymnasium.Env

Before learning how to create your own environment you should check out the documentation of Gymnasium’s API.

To illustrate the process of subclassing gymnasium.Env, we will implement a very simplistic game, called GridWorldEnv. We will write the code for our custom environment in gymnasium_env/envs/ The environment consists of a 2-dimensional square grid of fixed size (specified via the size parameter during construction). The agent can move vertically or horizontally between grid cells in each timestep. The goal of the agent is to navigate to a target on the grid that has been placed randomly at the beginning of the episode.

  • Observations provide the location of the target and agent.

  • There are 4 actions in our environment, corresponding to the movements “right”, “up”, “left”, and “down”.

  • A done signal is issued as soon as the agent has navigated to the grid cell where the target is located.

  • Rewards are binary and sparse, meaning that the immediate reward is always zero, unless the agent has reached the target, then it is 1.

An episode in this environment (with size=5) might look like this:

where the blue dot is the agent and the red square represents the target.

Let us look at the source code of GridWorldEnv piece by piece:

Declaration and Initialization

Our custom environment will inherit from the abstract class gymnasium.Env. You shouldn’t forget to add the metadata attribute to your class. There, you should specify the render-modes that are supported by your environment (e.g., "human", "rgb_array", "ansi") and the framerate at which your environment should be rendered. Every environment should support None as render-mode; you don’t need to add it in the metadata. In GridWorldEnv, we will support the modes “rgb_array” and “human” and render at 4 FPS.

The __init__ method of our environment will accept the integer size, that determines the size of the square grid. We will set up some variables for rendering and define self.observation_space and self.action_space. In our case, observations should provide information about the location of the agent and target on the 2-dimensional grid. We will choose to represent observations in the form of dictionaries with keys "agent" and "target". An observation may look like {"agent": array([1, 0]), "target": array([0, 3])}. Since we have 4 actions in our environment (“right”, “up”, “left”, “down”), we will use Discrete(4) as an action space. Here is the declaration of GridWorldEnv and the implementation of __init__:

# gymnasium_env/envs/
from enum import Enum

import numpy as np
import pygame

import gymnasium as gym
from gymnasium import spaces

class Actions(Enum):
    RIGHT = 0
    UP = 1
    LEFT = 2
    DOWN = 3

class GridWorldEnv(gym.Env):
    metadata = {"render_modes": ["human", "rgb_array"], "render_fps": 4}

    def __init__(self, render_mode=None, size=5):
        self.size = size  # The size of the square grid
        self.window_size = 512  # The size of the PyGame window

        # Observations are dictionaries with the agent's and the target's location.
        # Each location is encoded as an element of {0, ..., `size`}^2, i.e. MultiDiscrete([size, size]).
        self.observation_space = spaces.Dict(
                "agent": spaces.Box(0, size - 1, shape=(2,), dtype=int),
                "target": spaces.Box(0, size - 1, shape=(2,), dtype=int),
        self._agent_location = np.array([-1, -1], dtype=int)
        self._target_location = np.array([-1, -1], dtype=int)

        # We have 4 actions, corresponding to "right", "up", "left", "down"
        self.action_space = spaces.Discrete(4)

        The following dictionary maps abstract actions from `self.action_space` to
        the direction we will walk in if that action is taken.
        i.e. 0 corresponds to "right", 1 to "up" etc.
        self._action_to_direction = {
            Actions.RIGHT.value: np.array([1, 0]),
            Actions.UP.value: np.array([0, 1]),
            Actions.LEFT.value: np.array([-1, 0]),
            Actions.DOWN.value: np.array([0, -1]),

        assert render_mode is None or render_mode in self.metadata["render_modes"]
        self.render_mode = render_mode

        If human-rendering is used, `self.window` will be a reference
        to the window that we draw to. `self.clock` will be a clock that is used
        to ensure that the environment is rendered at the correct framerate in
        human-mode. They will remain `None` until human-mode is used for the
        first time.
        self.window = None
        self.clock = None

Constructing Observations From Environment States

Since we will need to compute observations both in reset and step, it is often convenient to have a (private) method _get_obs that translates the environment’s state into an observation. However, this is not mandatory and you may as well compute observations in reset and step separately:

def _get_obs(self):
    return {"agent": self._agent_location, "target": self._target_location}

We can also implement a similar method for the auxiliary information that is returned by step and reset. In our case, we would like to provide the manhattan distance between the agent and the target:

def _get_info(self):
    return {
        "distance": np.linalg.norm(
            self._agent_location - self._target_location, ord=1

Oftentimes, info will also contain some data that is only available inside the step method (e.g., individual reward terms). In that case, we would have to update the dictionary that is returned by _get_info in step.


The reset method will be called to initiate a new episode. You may assume that the step method will not be called before reset has been called. Moreover, reset should be called whenever a done signal has been issued. Users may pass the seed keyword to reset to initialize any random number generator that is used by the environment to a deterministic state. It is recommended to use the random number generator self.np_random that is provided by the environment’s base class, gymnasium.Env. If you only use this RNG, you do not need to worry much about seeding, but you need to remember to call ``super().reset(seed=seed)`` to make sure that gymnasium.Env correctly seeds the RNG. Once this is done, we can randomly set the state of our environment. In our case, we randomly choose the agent’s location and the random sample target positions, until it does not coincide with the agent’s position.

The reset method should return a tuple of the initial observation and some auxiliary information. We can use the methods _get_obs and _get_info that we implemented earlier for that:

def reset(self, seed=None, options=None):
    # We need the following line to seed self.np_random

    # Choose the agent's location uniformly at random
    self._agent_location = self.np_random.integers(0, self.size, size=2, dtype=int)

    # We will sample the target's location randomly until it does not coincide with the agent's location
    self._target_location = self._agent_location
    while np.array_equal(self._target_location, self._agent_location):
        self._target_location = self.np_random.integers(
            0, self.size, size=2, dtype=int

    observation = self._get_obs()
    info = self._get_info()

    if self.render_mode == "human":

    return observation, info


The step method usually contains most of the logic of your environment. It accepts an action, computes the state of the environment after applying that action and returns the 5-tuple (observation, reward, terminated, truncated, info). See gymnasium.Env.step(). Once the new state of the environment has been computed, we can check whether it is a terminal state and we set done accordingly. Since we are using sparse binary rewards in GridWorldEnv, computing reward is trivial once we know done.To gather observation and info, we can again make use of _get_obs and _get_info:

def step(self, action):
    # Map the action (element of {0,1,2,3}) to the direction we walk in
    direction = self._action_to_direction[action]
    # We use `np.clip` to make sure we don't leave the grid
    self._agent_location = np.clip(
        self._agent_location + direction, 0, self.size - 1
    # An episode is done iff the agent has reached the target
    terminated = np.array_equal(self._agent_location, self._target_location)
    reward = 1 if terminated else 0  # Binary sparse rewards
    observation = self._get_obs()
    info = self._get_info()

    if self.render_mode == "human":

    return observation, reward, terminated, False, info


Here, we are using PyGame for rendering. A similar approach to rendering is used in many environments that are included with Gymnasium and you can use it as a skeleton for your own environments:

def render(self):
    if self.render_mode == "rgb_array":
        return self._render_frame()

def _render_frame(self):
    if self.window is None and self.render_mode == "human":
        self.window = pygame.display.set_mode(
            (self.window_size, self.window_size)
    if self.clock is None and self.render_mode == "human":
        self.clock = pygame.time.Clock()

    canvas = pygame.Surface((self.window_size, self.window_size))
    canvas.fill((255, 255, 255))
    pix_square_size = (
        self.window_size / self.size
    )  # The size of a single grid square in pixels

    # First we draw the target
        (255, 0, 0),
            pix_square_size * self._target_location,
            (pix_square_size, pix_square_size),
    # Now we draw the agent
        (0, 0, 255),
        (self._agent_location + 0.5) * pix_square_size,
        pix_square_size / 3,

    # Finally, add some gridlines
    for x in range(self.size + 1):
            (0, pix_square_size * x),
            (self.window_size, pix_square_size * x),
            (pix_square_size * x, 0),
            (pix_square_size * x, self.window_size),

    if self.render_mode == "human":
        # The following line copies our drawings from `canvas` to the visible window
        self.window.blit(canvas, canvas.get_rect())

        # We need to ensure that human-rendering occurs at the predefined framerate.
        # The following line will automatically add a delay to keep the framerate stable.
    else:  # rgb_array
        return np.transpose(
            np.array(pygame.surfarray.pixels3d(canvas)), axes=(1, 0, 2)


The close method should close any open resources that were used by the environment. In many cases, you don’t actually have to bother to implement this method. However, in our example render_mode may be "human" and we might need to close the window that has been opened:

def close(self):
    if self.window is not None:

In other environments close might also close files that were opened or release other resources. You shouldn’t interact with the environment after having called close.

Registering Envs

In order for the custom environments to be detected by Gymnasium, they must be registered as follows. We will choose to put this code in gymnasium_env/

from gymnasium.envs.registration import register


The environment ID consists of three components, two of which are optional: an optional namespace (here: gymnasium_env), a mandatory name (here: GridWorld) and an optional but recommended version (here: v0). It might have also been registered as GridWorld-v0 (the recommended approach), GridWorld or gymnasium_env/GridWorld, and the appropriate ID should then be used during environment creation.

The keyword argument max_episode_steps=300 will ensure that GridWorld environments that are instantiated via gymnasium.make will be wrapped in a TimeLimit wrapper (see the wrapper documentation for more information). A done signal will then be produced if the agent has reached the target or 300 steps have been executed in the current episode. To distinguish truncation and termination, you can check info["TimeLimit.truncated"].

Apart from id and entrypoint, you may pass the following additional keyword arguments to register:








The reward threshold before the task is considered solved




Whether this environment is non-deterministic even after seeding




The maximum number of steps that an episode can consist of. If not None, a TimeLimit wrapper is added




Whether to wrap the environment in an OrderEnforcing wrapper




The default kwargs to pass to the environment class

Most of these keywords (except for max_episode_steps, order_enforce and kwargs) do not alter the behavior of environment instances but merely provide some extra information about your environment. After registration, our custom GridWorldEnv environment can be created with env = gymnasium.make('gymnasium_env/GridWorld-v0').

gymnasium_env/envs/ should have:

from gymnasium_env.envs.grid_world import GridWorldEnv

If your environment is not registered, you may optionally pass a module to import, that would register your environment before creating it like this - env = gymnasium.make('module:Env-v0'), where module contains the registration code. For the GridWorld env, the registration code is run by importing gymnasium_env so if it were not possible to import gymnasium_env explicitly, you could register while making by env = gymnasium.make('gymnasium_env:gymnasium_env/GridWorld-v0'). This is especially useful when you’re allowed to pass only the environment ID into a third-party codebase (eg. learning library). This lets you register your environment without needing to edit the library’s source code.

Creating a Package

The last step is to structure our code as a Python package. This involves configuring pyproject.toml. A minimal example of how to do so is as follows:

requires = ["hatchling"]
build-backend = ""

name = "gymnasium_env"
version = "0.0.1"
dependencies = [

Creating Environment Instances

Now you can install your package locally with:

pip install -e .

And you can create an instance of the environment via:


import gymnasium
import gymnasium_env
env = gymnasium.make('gymnasium_env/GridWorld-v0')

You can also pass keyword arguments of your environment’s constructor to gymnasium.make to customize the environment. In our case, we could do:

env = gymnasium.make('gymnasium_env/GridWorld-v0', size=10)

Sometimes, you may find it more convenient to skip registration and call the environment’s constructor yourself. Some may find this approach more pythonic and environments that are instantiated like this are also perfectly fine (but remember to add wrappers as well!).

Using Wrappers

Oftentimes, we want to use different variants of a custom environment, or we want to modify the behavior of an environment that is provided by Gymnasium or some other party. Wrappers allow us to do this without changing the environment implementation or adding any boilerplate code. Check out the wrapper documentation for details on how to use wrappers and instructions for implementing your own. In our example, observations cannot be used directly in learning code because they are dictionaries. However, we don’t actually need to touch our environment implementation to fix this! We can simply add a wrapper on top of environment instances to flatten observations into a single array:

import gymnasium
import gymnasium_env
from gymnasium.wrappers import FlattenObservation

env = gymnasium.make('gymnasium_env/GridWorld-v0')
wrapped_env = FlattenObservation(env)
print(wrapped_env.reset())     # E.g.  [3 0 3 3], {}

Wrappers have the big advantage that they make environments highly modular. For instance, instead of flattening the observations from GridWorld, you might only want to look at the relative position of the target and the agent. In the section on ObservationWrappers we have implemented a wrapper that does this job. This wrapper is also available in gymnasium_env/wrappers/

import gymnasium
import gymnasium_env
from gymnasium_env.wrappers import RelativePosition

env = gymnasium.make('gymnasium_env/GridWorld-v0')
wrapped_env = RelativePosition(env)
print(wrapped_env.reset())     # E.g.  [-3  3], {}