gym.Env.step(self, action: ActType) Tuple[ObsType, float, bool, bool, dict]#

Run one timestep of the environment’s dynamics.

When end of episode is reached, you are responsible for calling reset() to reset this environment’s state. Accepts an action and returns either a tuple (observation, reward, terminated, truncated, info).


action (ActType) – an action provided by the agent

  • observation (object) – this will be an element of the environment’s observation_space. This may, for instance, be a numpy array containing the positions and velocities of certain objects.

  • reward (float) – The amount of reward returned as a result of taking the action.

  • terminated (bool) – whether a terminal state (as defined under the MDP of the task) is reached. In this case further step() calls could return undefined results.

  • truncated (bool) – whether a truncation condition outside the scope of the MDP is satisfied. Typically a timelimit, but could also be used to indicate agent physically going out of bounds. Can be used to end the episode prematurely before a terminal state is reached.

  • info (dictionary) – info contains auxiliary diagnostic information (helpful for debugging, learning, and logging). This might, for instance, contain: metrics that describe the agent’s performance state, variables that are hidden from observations, or individual reward terms that are combined to produce the total reward. It also can contain information that distinguishes truncation and termination, however this is deprecated in favour of returning two booleans, and will be removed in a future version.

  • (deprecated)

  • done (bool) – A boolean value for if the episode has ended, in which case further step() calls will return undefined results. A done signal may be emitted for different reasons: Maybe the task underlying the environment was solved successfully, a certain timelimit was exceeded, or the physics simulation has entered an invalid state.

gym.Env.reset(self, *, seed: int | None = None, options: dict | None = None) Tuple[ObsType, dict]#

Resets the environment to an initial state and returns the initial observation.

This method can reset the environment’s random number generator(s) if seed is an integer or if the environment has not yet initialized a random number generator. If the environment already has a random number generator and reset() is called with seed=None, the RNG should not be reset. Moreover, reset() should (in the typical use case) be called with an integer seed right after initialization and then never again.

  • seed (optional int) – The seed that is used to initialize the environment’s PRNG. If the environment does not already have a PRNG and seed=None (the default option) is passed, a seed will be chosen from some source of entropy (e.g. timestamp or /dev/urandom). However, if the environment already has a PRNG and seed=None is passed, the PRNG will not be reset. If you pass an integer, the PRNG will be reset even if it already exists. Usually, you want to pass an integer right after the environment has been initialized and then never again. Please refer to the minimal example above to see this paradigm in action.

  • options (optional dict) – Additional information to specify how the environment is reset (optional, depending on the specific environment)

  • observation (object) – Observation of the initial state. This will be an element of observation_space (typically a numpy array) and is analogous to the observation returned by step().

  • info (dictionary) – This dictionary contains auxiliary information complementing observation. It should be analogous to the info returned by step().

gym.Env.render(self) RenderFrame | List[RenderFrame] | None#

Compute the render frames as specified by render_mode attribute during initialization of the environment.

The set of supported modes varies per environment. (And some third-party environments may not support rendering at all.) By convention, if render_mode is:

  • None (default): no render is computed.

  • human: render return None. The environment is continuously rendered in the current display or terminal. Usually for human consumption.

  • rgb_array: return a single frame representing the current state of the environment. A frame is a numpy.ndarray with shape (x, y, 3) representing RGB values for an x-by-y pixel image.

  • rgb_array_list: return a list of frames representing the states of the environment since the last reset. Each frame is a numpy.ndarray with shape (x, y, 3), as with rgb_array.

  • ansi: Return a strings (str) or StringIO.StringIO containing a terminal-style text representation for each time step. The text can include newlines and ANSI escape sequences (e.g. for colors).


Make sure that your class’s metadata ‘render_modes’ key includes the list of supported modes. It’s recommended to call super() in implementations to use the functionality of this method.


Env.action_space: Space[ActType]#

This attribute gives the format of valid actions. It is of datatype Space provided by Gym. For example, if the action space is of type Discrete and gives the value Discrete(2), this means there are two valid discrete actions: 0 & 1.

>>> env.action_space
>>> env.observation_space
Box(-3.4028234663852886e+38, 3.4028234663852886e+38, (4,), float32)
Env.observation_space: Space[ObsType]#

This attribute gives the format of valid observations. It is of datatype Space provided by Gym. For example, if the observation space is of type Box and the shape of the object is (4,), this denotes a valid observation will be an array of 4 numbers. We can check the box bounds as well with attributes.

>>> env.observation_space.high
array([4.8000002e+00, 3.4028235e+38, 4.1887903e-01, 3.4028235e+38], dtype=float32)
>>> env.observation_space.low
array([-4.8000002e+00, -3.4028235e+38, -4.1887903e-01, -3.4028235e+38], dtype=float32)
Env.reward_range = (-inf, inf)#

This attribute is a tuple corresponding to min and max possible rewards. Default range is set to (-inf,+inf). You can set it if you want a narrower range.

Additional Methods#


Override close in your subclass to perform any necessary cleanup.

Environments will automatically close() themselves when garbage collected or when the program exits.


class gym.Wrapper(env: Env)#

Wraps an environment to allow a modular transformation of the step() and reset() methods.

This class is the base class for all wrappers. The subclass could override some methods to change the behavior of the original environment without touching the original code.


Don’t forget to call super().__init__(env) if the subclass overrides __init__().


class gym.ObservationWrapper(env: Env)#

Superclass of wrappers that can modify observations using observation() for reset() and step().

If you would like to apply a function to the observation that is returned by the base environment before passing it to learning code, you can simply inherit from ObservationWrapper and overwrite the method observation() to implement that transformation. The transformation defined in that method must be defined on the base environment’s observation space. However, it may take values in a different space. In that case, you need to specify the new observation space of the wrapper by setting self.observation_space in the __init__() method of your wrapper.

For example, you might have a 2D navigation task where the environment returns dictionaries as observations with keys "agent_position" and "target_position". A common thing to do might be to throw away some degrees of freedom and only consider the position of the target relative to the agent, i.e. observation["target_position"] - observation["agent_position"]. For this, you could implement an observation wrapper like this:

class RelativePosition(gym.ObservationWrapper):
    def __init__(self, env):
        self.observation_space = Box(shape=(2,), low=-np.inf, high=np.inf)

    def observation(self, obs):
        return obs["target"] - obs["agent"]

Among others, Gym provides the observation wrapper TimeAwareObservation, which adds information about the index of the timestep to the observation.


class gym.RewardWrapper(env: Env)#

Superclass of wrappers that can modify the returning reward from a step.

If you would like to apply a function to the reward that is returned by the base environment before passing it to learning code, you can simply inherit from RewardWrapper and overwrite the method reward() to implement that transformation. This transformation might change the reward range; to specify the reward range of your wrapper, you can simply define self.reward_range in __init__().

Let us look at an example: Sometimes (especially when we do not have control over the reward because it is intrinsic), we want to clip the reward to a range to gain some numerical stability. To do that, we could, for instance, implement the following wrapper:

class ClipReward(gym.RewardWrapper):
    def __init__(self, env, min_reward, max_reward):
        self.min_reward = min_reward
        self.max_reward = max_reward
        self.reward_range = (min_reward, max_reward)

    def reward(self, reward):
        return np.clip(reward, self.min_reward, self.max_reward)


class gym.ActionWrapper(env: Env)#

Superclass of wrappers that can modify the action before env.step().

If you would like to apply a function to the action before passing it to the base environment, you can simply inherit from ActionWrapper and overwrite the method action() to implement that transformation. The transformation defined in that method must take values in the base environment’s action space. However, its domain might differ from the original action space. In that case, you need to specify the new action space of the wrapper by setting self.action_space in the __init__() method of your wrapper.

Let’s say you have an environment with action space of type gym.spaces.Box, but you would only like to use a finite subset of actions. Then, you might want to implement the following wrapper:

class DiscreteActions(gym.ActionWrapper):
    def __init__(self, env, disc_to_cont):
        self.disc_to_cont = disc_to_cont
        self.action_space = Discrete(len(disc_to_cont))

    def action(self, act):
        return self.disc_to_cont[act]

if __name__ == "__main__":
    env = gym.make("LunarLanderContinuous-v2")
    wrapped_env = DiscreteActions(env, [np.array([1,0]), np.array([-1,0]),
                                        np.array([0,1]), np.array([0,-1])])
    print(wrapped_env.action_space)         #Discrete(4)

Among others, Gym provides the action wrappers ClipAction and RescaleAction.