Allow frame stack size of 1 in FrameStack wrapper (#1192)

Author: JDRanpariya

gymnasium/wrappers/stateful_observation.py

@@ -372,9 +372,9 @@ def __init__(
             raise TypeError(
                 f"The stack_size is expected to be an integer, actual type: {type(stack_size)}"
             )
-        if not 1 < stack_size:
+        if not 0 < stack_size:
             raise ValueError(
-                f"The stack_size needs to be greater than one, actual value: {stack_size}"
+                f"The stack_size needs to be greater than zero, actual value: {stack_size}"
             )
         if isinstance(padding_type, str) and (
             padding_type == "reset" or padding_type == "zero"

tests/wrappers/test_frame_stack_observation.py

@@ -134,9 +134,11 @@ def test_stack_size_failures():
 
     with pytest.raises(
         ValueError,
-        match=re.escape("The stack_size needs to be greater than one, actual value: 1"),
+        match=re.escape(
+            "The stack_size needs to be greater than zero, actual value: 0"
+        ),
     ):
-        FrameStackObservation(env, stack_size=1)
+        FrameStackObservation(env, stack_size=0)
 
     with pytest.raises(
         ValueError,