Changed .shape to be a property (openai#2397)

* Changed .shape to a property, and the internal representation of a shape to ._shape

* Minor docstring fix

gym/spaces/box.py

@@ -57,7 +57,7 @@ def __init__(self, low, high, shape=None, dtype=np.float32):
         if np.isscalar(high):
             high = np.full(shape, high, dtype=dtype)
 
-        self.shape = shape
+        self._shape = shape
         self.low = low
         self.high = high
 

gym/spaces/space.py

@@ -19,7 +19,7 @@ class Space(object):
     def __init__(self, shape=None, dtype=None):
         import numpy as np  # takes about 300-400ms to import, so we load lazily
 
-        self.shape = None if shape is None else tuple(shape)
+        self._shape = None if shape is None else tuple(shape)
         self.dtype = None if dtype is None else np.dtype(dtype)
         self._np_random = None
 
@@ -33,6 +33,11 @@ def np_random(self):
 
         return self._np_random
 
+    @property
+    def shape(self):
+        """Return the shape of the space as an immutable property"""
+        return self._shape
+
     def sample(self):
         """Randomly sample an element of this space. Can be
         uniform or non-uniform sampling based on boundedness of space."""