Add explicit error messages when unflatten discrete and multidiscrete…

… fail (openai#267)

gymnasium/spaces/utils.py

@@ -271,7 +271,13 @@ def _unflatten_box_multibinary(
 
 @unflatten.register(Discrete)
 def _unflatten_discrete(space: Discrete, x: NDArray[np.int64]) -> np.int64:
-    return space.start + np.nonzero(x)[0][0]
+    nonzero = np.nonzero(x)
+    if len(nonzero[0]) == 0:
+        raise ValueError(
+            f"{x} is not a valid one-hot encoded vector and can not be unflattened to space {space}. "
+            "Not all valid samples in a flattened space can be unflattened."
+        )
+    return space.start + nonzero[0][0]
 
 
 @unflatten.register(MultiDiscrete)
@@ -280,8 +286,13 @@ def _unflatten_multidiscrete(
 ) -> NDArray[np.integer[Any]]:
     offsets = np.zeros((space.nvec.size + 1,), dtype=space.dtype)
     offsets[1:] = np.cumsum(space.nvec.flatten())
-
-    (indices,) = cast(type(offsets[:-1]), np.nonzero(x))
+    nonzero = np.nonzero(x)
+    if len(nonzero[0]) == 0:
+        raise ValueError(
+            f"{x} is not a concatenation of one-hot encoded vectors and can not be unflattened to space {space}. "
+            "Not all valid samples in a flattened space can be unflattened."
+        )
+    (indices,) = cast(type(offsets[:-1]), nonzero)
     return np.asarray(indices - offsets[:-1], dtype=space.dtype).reshape(space.shape)
 
 

tests/spaces/test_utils.py

@@ -135,3 +135,15 @@ def test_flatten_roundtripping(space):
 
     for original, roundtripped in zip(samples, unflattened_samples):
         assert data_equivalence(original, roundtripped)
+
+
+def test_unflatten_discrete_error():
+    value = np.array([0])
+    with pytest.raises(ValueError):
+        utils.unflatten(gym.spaces.Discrete(1), value)
+
+
+def test_unflatten_multidiscrete_error():
+    value = np.array([0, 0])
+    with pytest.raises(ValueError):
+        utils.unflatten(gym.spaces.MultiDiscrete([1, 1]), value)