Serializing a custom space#

In this tutorial you’ll learn how to serialize a custom Gymnasium observation space and use that to create a Minari dataset with minari.DataCollector. We’ll use the MiniGrid Empty environment and show how to serialize its unique observation space.

Serializing a custom space can be applied to both observation and action spaces.

Let’s start by installing the minigrid library:

pip install minigrid

Then we can import the required modules:

import json
from typing import Dict, Union

import gymnasium as gym
from minigrid.core.mission import MissionSpace

import minari
from minari import DataCollector
from minari.serialization import deserialize_space, serialize_space

First we’ll initialize the MiniGrid Empty environment and take a look at its observation and action space.

env = gym.make("MiniGrid-Empty-16x16-v0")

print(f"Observation space: {env.observation_space}")
print(f"Action space: {env.action_space}")

We can see the output from above looks like:

Observation space: Dict(‘direction’: Discrete(4), ‘image’: Box(0, 255, (7, 7, 3), uint8), ‘mission’: MissionSpace(<function EmptyEnv._gen_mission at 0x12253a940>, None))

Action space: Discrete(7)

If we take a look at Minari’s serialization functions we can see that Dict, Discrete, and Box are all supported. However MissionSpace is not supported and if try to serialize it with:


Then we will encounter a NotImplementedError error:

NotImplementedError: No serialization method available for MissionSpace(<function EmptyEnv._gen_mission at 0x12253a940>, None)

But what is MissonSpace? If we look at the source code we can see that it is simply a wrapper around a Callable that returns a randomly generated string representing the environment’s mission. Let’s sample from the mission space to see an example:


This will print out:

‘get to the green goal square’

For this particular environment we don’t have to worry about the mission string varying from sample to sample.

Now that we have a custom observation space we need to define functions that properly serialize and deserialize it.

When creating a Minari dataset, the space data gets serialized to a JSON format before saving to disk. The serialize_space function takes care of this conversion for various supported Gymnasium spaces. To enable serialization for a custom space we can register 2 new functions that will serialize the space into a JSON object and also deserialize it back into the custom space.

def serialize_custom_space(space: MissionSpace, to_string=True) -> Union[Dict, str]:
    result = {}
    result["type"] = "MissionSpace"
    result["mission_func"] = space.mission_func()

    if to_string:
        result = json.dumps(result)
    return result

def deserialize_custom_space(space_dict: Dict) -> MissionSpace:
    assert space_dict["type"] == "MissionSpace"
    mission_func = lambda: space_dict["mission_func"]  # noqa: E731

    return MissionSpace(

Now that we have serialization functions for MissionSpace we can collect some episode data.

dataset_id = "minigrid-custom-space-v0"

env = DataCollector(env)
num_episodes = 10


for episode in range(num_episodes):
    terminated = False
    truncated = False
    while not terminated and not truncated:
        action = env.action_space.sample()  # Choose random actions
        _, _, terminated, truncated, _ = env.step(action)

Finally we can create a Minari dataset.

dataset = env.create_dataset(

To show that the custom space was properly serialized we can load the dataset we just created and take a look at the observation space.

del dataset
dataset = minari.load_dataset(dataset_id)


The output should show the original observation space from earlier except with a different MissionSpace function name since we created it inside deserialize_custom_space:

Dict(‘direction’: Discrete(4), ‘image’: Box(0, 255, (7, 7, 3), uint8), ‘mission’: MissionSpace(<function deserialize_custom_space.<locals>.<lambda> at 0x11f2608b0>, None))

Finally to clean things up, we’ll delete the dataset we created earlier: