Repository: openai/gym

Issue:
API Compliance Testing
To help users with creating custom environments, we should have a user test for environments to ensure the Gym API is properly followed, similar to what PettingZoo has. The easiest solution is to upstream the checker in SB3.

Crosspost from https://github.com/DLR-RM/stable-baselines3/issues/525


Discussion:
I'd be interested in picking this up if nobody else has started work on it!
Add far as I know no one has yet :)
Great, I'll get working on it then!
quick remark: i don't mind SB3 env checker being copied but please don't forget to comply to the MIT license ;) (explicit mention of the original authors/license + link to the repo in the file)

Resolving patch:
diff --git a/gym/utils/env_checker.py b/gym/utils/env_checker.py
new file mode 100644
--- /dev/null
+++ b/gym/utils/env_checker.py
@@ -0,0 +1,334 @@
+"""
+This file is originally from the Stable Baselines3 repository hosted on GitHub
+(https://github.com/DLR-RM/stable-baselines3/)
+Original Author: Antonin Raffin
+
+It also uses some warnings/assertions from the PettingZoo repository hosted on GitHub
+(https://github.com/PettingZoo-Team/PettingZoo)
+Original Author: Justin Terry
+
+These projects are covered by the MIT License.
+"""
+
+import warnings
+from typing import Union
+
+import gym
+import numpy as np
+from gym import spaces
+
+
+def _is_numpy_array_space(space: spaces.Space) -> bool:
+    """
+    Returns False if provided space is not representable as a single numpy array
+    (e.g. Dict and Tuple spaces return False)
+    """
+    return not isinstance(space, (spaces.Dict, spaces.Tuple))
+
+
+def _check_image_input(observation_space: spaces.Box, key: str = "") -> None:
+    """
+    Check that the input adheres to general standards
+    when the observation is apparently an image.
+    """
+    if observation_space.dtype != np.uint8:
+        warnings.warn(
+            f"It seems that your observation {key} is an image but the `dtype` "
+            "of your observation_space is not `np.uint8`. "
+            "If your observation is not an image, we recommend you to flatten the observation "
+            "to have only a 1D vector"
+        )
+
+    if np.any(observation_space.low != 0) or np.any(observation_space.high != 255):
+        warnings.warn(
+            f"It seems that your observation space {key} is an image but the "
+            "upper and lower bounds are not in [0, 255]. "
+            "Generally, CNN policies assume observations are within that range, "
+            "so you may encounter an issue if the observation values are not."
+        )
+
+
+def _check_nan(env: gym.Env, check_inf: bool = True) -> None:
+    """Check for NaN and Inf."""
+    for _ in range(10):
+        action = env.action_space.sample()
+        observation, reward, _, _ = env.step(action)
+
+        if np.any(np.isnan(observation)):
+            warnings.warn("Encountered NaN value in observations.")
+        if np.any(np.isnan(reward)):
+            warnings.warn("Encountered NaN value in rewards.")
+        if check_inf and np.any(np.isinf(observation)):
+            warnings.warn("Encountered inf value in observations.")
+        if check_inf and np.any(np.isinf(reward)):
+            warnings.warn("Encountered inf value in rewards.")
+
+
+def _check_obs(
+    obs: Union[tuple, dict, np.ndarray, int],
+    observation_space: spaces.Space,
+    method_name: str,
+) -> None:
+    """
+    Check that the observation returned by the environment
+    correspond to the declared one.
+    """
+    if not isinstance(observation_space, spaces.Tuple):
+        assert not isinstance(
+            obs, tuple
+        ), f"The observation returned by the `{method_name}()` method should be a single value, not a tuple"
+
+    # The check for a GoalEnv is done by the base class
+    if isinstance(observation_space, spaces.Discrete):
+        assert isinstance(
+            obs, int
+        ), f"The observation returned by `{method_name}()` method must be an int"
+    elif _is_numpy_array_space(observation_space):
+        assert isinstance(
+            obs, np.ndarray
+        ), f"The observation returned by `{method_name}()` method must be a numpy array"
+
+    assert observation_space.contains(
+        obs
+    ), f"The observation returned by the `{method_name}()` method does not match the given observation space"
+
+
+def _check_box_obs(observation_space: spaces.Box, key: str = "") -> None:
+    """
+    Check that the observation space is correctly formatted
+    when dealing with a ``Box()`` space. In particular, it checks:
+    - that the dimensions are big enough when it is an image, and that the type matches
+    - that the observation has an expected shape (warn the user if not)
+    """
+    # If image, check the low and high values, the type and the number of channels
+    # and the shape (minimal value)
+    if len(observation_space.shape) == 3:
+        _check_image_input(observation_space)
+
+    if len(observation_space.shape) not in [1, 3]:
+        warnings.warn(
+            f"Your observation {key} has an unconventional shape (neither an image, nor a 1D vector). "
+            "We recommend you to flatten the observation "
+            "to have only a 1D vector or use a custom policy to properly process the data."
+        )
+
+    if np.any(np.equal(observation_space.low, -np.inf)):
+        warnings.warn(
+            "Agent's minimum observation space value is -infinity. This is probably too low."
+        )
+    if np.any(np.equal(observation_space.high, np.inf)):
+        warnings.warn(
+            "Agent's maxmimum observation space value is infinity. This is probably too high"
+        )
+    if np.any(np.equal(observation_space.low, observation_space.high)):
+        warnings.warn("Agent's maximum and minimum observation space values are equal")
+    if np.any(np.greater(observation_space.low, observation_space.high)):
+        assert False, "Agent's minimum observation value is greater than it's maximum"
+    if observation_space.low.shape != observation_space.shape:
+        assert (
+            False
+        ), "Agent's observation_space.low and observation_space have different shapes"
+    if observation_space.high.shape != observation_space.shape:
+        assert (
+            False
+        ), "Agent's observation_space.high and observation_space have different shapes"
+
+
+def _check_box_action(action_space: spaces.Box):
+    if np.any(np.equal(action_space.low, -np.inf)):
+        warnings.warn(
+            "Agent's minimum action space value is -infinity. This is probably too low."
+        )
+    if np.any(np.equal(action_space.high, np.inf)):
+        warnings.warn(
+            "Agent's maxmimum action space value is infinity. This is probably too high"
+        )
+    if np.any(np.equal(action_space.low, action_space.high)):
+        warnings.warn("Agent's maximum and minimum action space values are equal")
+    if np.any(np.greater(action_space.low, action_space.high)):
+        assert False, "Agent's minimum action value is greater than it's maximum"
+    if action_space.low.shape != action_space.shape:
+        assert False, "Agent's action_space.low and action_space have different shapes"
+    if action_space.high.shape != action_space.shape:
+        assert False, "Agent's action_space.high and action_space have different shapes"
+
+
+def _check_normalized_action(action_space: spaces.Box):
+    if (
+        np.any(np.abs(action_space.low) != np.abs(action_space.high))
+        or np.any(np.abs(action_space.low) > 1)
+        or np.any(np.abs(action_space.high) > 1)
+    ):
+        warnings.warn(
+            "We recommend you to use a symmetric and normalized Box action space (range=[-1, 1]) "
+            "cf https://stable-baselines3.readthedocs.io/en/master/guide/rl_tips.html"
+        )
+
+
+def _check_returned_values(
+    env: gym.Env, observation_space: spaces.Space, action_space: spaces.Space
+) -> None:
+    """
+    Check the returned values by the env when calling `.reset()` or `.step()` methods.
+    """
+    # because env inherits from gym.Env, we assume that `reset()` and `step()` methods exists
+    obs = env.reset()
+
+    if isinstance(observation_space, spaces.Dict):
+        assert isinstance(
+            obs, dict
+        ), "The observation returned by `reset()` must be a dictionary"
+        for key in observation_space.spaces.keys():
+            try:
+                _check_obs(obs[key], observation_space.spaces[key], "reset")
+            except AssertionError as e:
+                raise AssertionError(f"Error while checking key={key}: " + str(e))
+    else:
+        _check_obs(obs, observation_space, "reset")
+
+    # Sample a random action
+    action = action_space.sample()
+    data = env.step(action)
+
+    assert (
+        len(data) == 4
+    ), "The `step()` method must return four values: obs, reward, done, info"
+
+    # Unpack
+    obs, reward, done, info = data
+
+    if isinstance(observation_space, spaces.Dict):
+        assert isinstance(
+            obs, dict
+        ), "The observation returned by `step()` must be a dictionary"
+        for key in observation_space.spaces.keys():
+            try:
+                _check_obs(obs[key], observation_space.spaces[key], "step")
+            except AssertionError as e:
+                raise AssertionError(f"Error while checking key={key}: " + str(e))
+
+    else:
+        _check_obs(obs, observation_space, "step")
+
+    # We also allow int because the reward will be cast to float
+    assert isinstance(
+        reward, (float, int, np.float32)
+    ), "The reward returned by `step()` must be a float"
+    assert isinstance(done, bool), "The `done` signal must be a boolean"
+    assert isinstance(
+        info, dict
+    ), "The `info` returned by `step()` must be a python dictionary"
+
+    if isinstance(env, gym.GoalEnv):
+        # For a GoalEnv, the keys are checked at reset
+        assert reward == env.compute_reward(
+            obs["achieved_goal"], obs["desired_goal"], info
+        )
+
+
+def _check_spaces(env: gym.Env) -> None:
+    """
+    Check that the observation and action spaces are defined
+    and inherit from gym.spaces.Space.
+    """
+    # Helper to link to the code, because gym has no proper documentation
+    gym_spaces = " cf https://github.com/openai/gym/blob/master/gym/spaces/"
+
+    assert hasattr(env, "observation_space"), (
+        "You must specify an observation space (cf gym.spaces)" + gym_spaces
+    )
+    assert hasattr(env, "action_space"), (
+        "You must specify an action space (cf gym.spaces)" + gym_spaces
+    )
+
+    assert isinstance(env.observation_space, spaces.Space), (
+        "The observation space must inherit from gym.spaces" + gym_spaces
+    )
+    assert isinstance(env.action_space, spaces.Space), (
+        "The action space must inherit from gym.spaces" + gym_spaces
+    )
+
+
+# Check render cannot be covered by CI
+def _check_render(
+    env: gym.Env, warn: bool = True, headless: bool = False
+) -> None:  # pragma: no cover
+    """
+    Check the declared render modes and the `render()`/`close()`
+    method of the environment.
+    :param env: The environment to check
+    :param warn: Whether to output additional warnings
+    :param headless: Whether to disable render modes
+        that require a graphical interface. False by default.
+    """
+    render_modes = env.metadata.get("render.modes")
+    if render_modes is None:
+        if warn:
+            warnings.warn(
+                "No render modes was declared in the environment "
+                " (env.metadata['render.modes'] is None or not defined), "
+                "you may have trouble when calling `.render()`"
+            )
+
+    else:
+        # Don't check render mode that require a
+        # graphical interface (useful for CI)
+        if headless and "human" in render_modes:
+            render_modes.remove("human")
+        # Check all declared render modes
+        for render_mode in render_modes:
+            env.render(mode=render_mode)
+        env.close()
+
+
+def check_env(env: gym.Env, warn: bool = True, skip_render_check: bool = True) -> None:
+    """
+    Check that an environment follows Gym API.
+    This is particularly useful when using a custom environment.
+    Please take a look at https://github.com/openai/gym/blob/master/gym/core.py
+    for more information about the API.
+    It also optionally check that the environment is compatible with Stable-Baselines.
+    :param env: The Gym environment that will be checked
+    :param warn: Whether to output additional warnings
+        mainly related to the interaction with Stable Baselines
+    :param skip_render_check: Whether to skip the checks for the render method.
+        True by default (useful for the CI)
+    """
+    assert isinstance(
+        env, gym.Env
+    ), "Your environment must inherit from the gym.Env class cf https://github.com/openai/gym/blob/master/gym/core.py"
+
+    # ============= Check the spaces (observation and action) ================
+    _check_spaces(env)
+
+    # Define aliases for convenience
+    observation_space = env.observation_space
+    action_space = env.action_space
+
+    # Warn the user if needed.
+    # A warning means that the environment may run but not work properly with popular RL libraries.
+    if warn:
+        obs_spaces = (
+            observation_space.spaces
+            if isinstance(observation_space, spaces.Dict)
+            else {"": observation_space}
+        )
+        for key, space in obs_spaces.items():
+            if isinstance(space, spaces.Box):
+                _check_box_obs(space, key)
+
+        # Check for the action space, it may lead to hard-to-debug issues
+        if isinstance(action_space, spaces.Box):
+            _check_box_action(action_space)
+            _check_normalized_action(action_space)
+
+    # ============ Check the returned values ===============
+    _check_returned_values(env, observation_space, action_space)
+
+    # ==== Check the render method and the declared render modes ====
+    if not skip_render_check:
+        _check_render(env, warn=warn)  # pragma: no cover
+
+    # The check only works with numpy arrays
+    if _is_numpy_array_space(observation_space) and _is_numpy_array_space(action_space):
+        _check_nan(env)


Test patch:
diff --git a/gym/envs/tests/test_envs.py b/gym/envs/tests/test_envs.py
--- a/gym/envs/tests/test_envs.py
+++ b/gym/envs/tests/test_envs.py
@@ -3,6 +3,7 @@
 
 from gym import envs
 from gym.envs.tests.spec_list import spec_list
+from gym.utils.env_checker import check_env
 
 
 # This runs a smoketest on each official registered env. We may want
@@ -14,6 +15,9 @@ def test_env(spec):
     with pytest.warns(None) as warnings:
         env = spec.make()
 
+    # Test if env adheres to Gym API
+    check_env(env, warn=True, skip_render_check=True)
+
     # Check that dtype is explicitly declared for gym.Box spaces
     for warning_msg in warnings:
         assert "autodetected dtype" not in str(warning_msg.message)
diff --git a/gym/utils/tests/test_env_checker.py b/gym/utils/tests/test_env_checker.py
new file mode 100644
--- /dev/null
+++ b/gym/utils/tests/test_env_checker.py
@@ -0,0 +1,35 @@
+import gym
+import numpy as np
+import pytest
+from gym.spaces import Box, Dict, Discrete
+
+from gym.utils.env_checker import check_env
+
+
+class ActionDictTestEnv(gym.Env):
+    action_space = Dict({"position": Discrete(1), "velocity": Discrete(1)})
+    observation_space = Box(low=-1.0, high=2.0, shape=(3,), dtype=np.float32)
+
+    def step(self, action):
+        observation = np.array([1.0, 1.5, 0.5])
+        reward = 1
+        done = True
+        return observation, reward, done
+
+    def reset(self):
+        return np.array([1.0, 1.5, 0.5])
+
+    def render(self, mode="human"):
+        pass
+
+
+def test_check_env_dict_action():
+    # Environment.step() only returns 3 values: obs, reward, done. Not info!
+    test_env = ActionDictTestEnv()
+
+    with pytest.raises(AssertionError) as errorinfo:
+        check_env(env=test_env, warn=True)
+        assert (
+            str(errorinfo.value)
+            == "The `step()` method must return four values: obs, reward, done, info"
+        )