Readded overwritten changes for offset functionality for Discrete spaces (#2470)

Co-authored-by: J K Terry <justinkterry@gmail.com>

docs/api.md

@@ -59,7 +59,7 @@ print(env.observation_space.low)
 ```
 - There are multiple types of Space types inherently available in gym:
 	- `Box` describes an n-dimensional continuous space. Its a bounded space where we can define the upper and lower limit which describe the valid values our observations can take.
-	- `Discrete` describes a discrete space where { 0, 1, ......., n-1} are the possible values our observation/action can take. 
+	- `Discrete` describes a discrete space where { 0, 1, ......., n-1} are the possible values our observation/action can take. Values can be shifted to { a, a+1, ......., a+n-1} using an optional argument.
 	- `Dict` represents a dictionary of simple spaces.
 	- `Tuple` represents a tuple of simple spaces
 	- `MultiBinary` creates a n-shape binary space. Argument n can be a number or a `list` of numbers
@@ -73,6 +73,10 @@ print(env.observation_space.low)
 	print(observation_space.sample())
 	#> 1
  
+	observation_space = Discrete(5, start=-2)
+	print(observation_space.sample())
+	#> -2
+
 	observation_space = Dict({"position": Discrete(2), "velocity": Discrete(3)})
 	print(observation_space.sample())
 	#> OrderedDict([('position', 0), ('velocity', 1)])

gym/envs/classic_control/pendulum.py

@@ -6,6 +6,7 @@ from os import path
 
 
 class PendulumEnv(gym.Env):
+
     metadata = {"render.modes": ["human", "rgb_array"], "video.frames_per_second": 30}
 
     def __init__(self, g=10.0):

gym/spaces/discrete.py

@@ -5,19 +5,24 @@ from .space import Space
 class Discrete(Space):
     r"""A discrete space in :math:`\{ 0, 1, \\dots, n-1 \}`.
 
+    A start value can be optionally specified to shift the range
+    to :math:`\{ a, a+1, \\dots, a+n-1 \}`.
+
     Example::
 
         >>> Discrete(2)
+        >>> Discrete(3, start=-1)  # {-1, 0, 1}
 
     """
 
-    def __init__(self, n, seed=None):
-        assert n >= 0
+    def __init__(self, n, seed=None, start=0):
+        assert n >= 0 and isinstance(start, (int, np.integer))
         self.n = n
+        self.start = int(start)
         super(Discrete, self).__init__((), np.int64, seed)
 
     def sample(self):
-        return self.np_random.randint(self.n)
+        return self.start + self.np_random.randint(self.n)
 
     def contains(self, x):
         if isinstance(x, int):
@@ -28,10 +33,16 @@ class Discrete(Space):
             as_int = int(x)
         else:
             return False
-        return as_int >= 0 and as_int < self.n
+        return self.start <= as_int < self.start + self.n
 
     def __repr__(self):
+        if self.start != 0:
+            return "Discrete(%d, start=%d)" % (self.n, self.start)
         return "Discrete(%d)" % self.n
 
     def __eq__(self, other):
-        return isinstance(other, Discrete) and self.n == other.n
+        return (
+            isinstance(other, Discrete)
+            and self.n == other.n
+            and self.start == other.start
+        )

tests/spaces/test_spaces.py

@@ -11,6 +11,7 @@ from gym.spaces import Tuple, Box, Discrete, MultiDiscrete, MultiBinary, Dict
     "space",
     [
         Discrete(3),
+        Discrete(5, start=-2),
         Box(low=0.0, high=np.inf, shape=(2, 2)),
         Tuple([Discrete(5), Discrete(10)]),
         Tuple(
@@ -20,6 +21,7 @@ from gym.spaces import Tuple, Box, Discrete, MultiDiscrete, MultiBinary, Dict
             ]
         ),
         Tuple((Discrete(5), Discrete(2), Discrete(2))),
+        Tuple((Discrete(5), Discrete(2, start=6), Discrete(2, start=-4))),
         MultiDiscrete([2, 2, 100]),
         MultiBinary(10),
         Dict(
@@ -56,6 +58,7 @@ def test_roundtripping(space):
     "space",
     [
         Discrete(3),
+        Discrete(5, start=-2),
         Box(low=np.array([-10, 0]), high=np.array([10, 10]), dtype=np.float32),
         Box(low=-np.inf, high=np.inf, shape=(1, 3)),
         Tuple([Discrete(5), Discrete(10)]),
@@ -66,6 +69,7 @@ def test_roundtripping(space):
             ]
         ),
         Tuple((Discrete(5), Discrete(2), Discrete(2))),
+        Tuple((Discrete(5), Discrete(2), Discrete(2, start=-6))),
         MultiDiscrete([2, 2, 100]),
         MultiBinary(6),
         Dict(
@@ -88,6 +92,7 @@ def test_equality(space):
     "spaces",
     [
         (Discrete(3), Discrete(4)),
+        (Discrete(3), Discrete(3, start=-1)),
         (MultiDiscrete([2, 2, 100]), MultiDiscrete([2, 2, 8])),
         (MultiBinary(8), MultiBinary(7)),
         (
@@ -99,6 +104,10 @@ def test_equality(space):
             Box(low=0.0, high=np.inf, shape=(2, 1)),
         ),
         (Tuple([Discrete(5), Discrete(10)]), Tuple([Discrete(1), Discrete(10)])),
+        (
+            Tuple([Discrete(5), Discrete(10)]),
+            Tuple([Discrete(5, start=7), Discrete(10)]),
+        ),
         (Dict({"position": Discrete(5)}), Dict({"position": Discrete(4)})),
         (Dict({"position": Discrete(5)}), Dict({"speed": Discrete(5)})),
     ],
@@ -112,6 +121,7 @@ def test_inequality(spaces):
     "space",
     [
         Discrete(5),
+        Discrete(8, start=-20),
         Box(low=0, high=255, shape=(2,), dtype="uint8"),
         Box(low=-np.inf, high=np.inf, shape=(3, 3)),
         Box(low=1.0, high=np.inf, shape=(3, 3)),
@@ -133,7 +143,7 @@ def test_sample(space):
         else:
             expected_mean = 0.0
     elif isinstance(space, Discrete):
-        expected_mean = space.n / 2
+        expected_mean = space.start + space.n / 2
     else:
         raise NotImplementedError
     np.testing.assert_allclose(expected_mean, samples.mean(), atol=3.0 * samples.std())
@@ -246,6 +256,7 @@ def test_box_dtype_check():
     "space",
     [
         Discrete(3),
+        Discrete(3, start=-4),
         Box(low=0.0, high=np.inf, shape=(2, 2)),
         Tuple([Discrete(5), Discrete(10)]),
         Tuple(
@@ -298,6 +309,7 @@ def sample_equal(sample1, sample2):
     "space",
     [
         Discrete(3),
+        Discrete(3, start=-4),
         Box(low=0.0, high=np.inf, shape=(2, 2)),
         Tuple([Discrete(5), Discrete(10)]),
         Tuple(
@@ -335,6 +347,7 @@ def test_seed_reproducibility(space):
     [
         Tuple([Discrete(100), Discrete(100)]),
         Tuple([Discrete(5), Discrete(10)]),
+        Tuple([Discrete(5), Discrete(5, start=10)]),
         Tuple(
             [
                 Discrete(5),