Never return None from Physics.bind.

Physics.bind used to handle `mjcf_elements == None` by returning None. For clients using type annotations, this made the API annoying, as the None case had to be handled.

After this commit, passing None will lead to a crash.

PiperOrigin-RevId: 613142103
Change-Id: I2ce3a98dddb61400202c5b2cd7fc67810e44c487

Author: nimrod-gileadi

dm_control/mjcf/physics.py

@@ -512,7 +512,7 @@ def forward(self):
     super().forward()
     self._dirty = False
 
-  def bind(self, mjcf_elements):
+  def bind(self, mjcf_elements) -> Binding:
     """Creates a binding between this `Physics` instance and `mjcf.Element`s.
 
     The binding allows for easier interaction with the `Physics` data structures
@@ -609,7 +609,7 @@ def bind(self, mjcf_elements):
       ValueError: If `mjcf_elements` cannot be bound to this Physics.
     """
     if mjcf_elements is None:
-      return None
+      raise ValueError('mjcf_elements is None.')
 
     # To reduce overhead from processing MJCF elements and making new bindings,
     # we cache and reuse existing Binding objects. The cheapest version of
@@ -625,13 +625,11 @@ def bind(self, mjcf_elements):
       # `mjcf_elements` is not iterable.
       cache_key = mjcf_elements
 
-    needs_new_binding = False
     try:
-      binding = self._bindings[cache_key]
+      return self._bindings[cache_key]
     except KeyError:
       # This means `mjcf_elements` is hashable, so we use it as cache key.
       namespace, named_index = names_from_elements(mjcf_elements)
-      needs_new_binding = True
     except TypeError:
       # This means `mjcf_elements` is unhashable, fallback to caching by name.
       namespace, named_index = names_from_elements(mjcf_elements)
@@ -644,14 +642,12 @@ def bind(self, mjcf_elements):
         cache_key = (namespace, named_index)
 
       try:
-        binding = self._bindings[cache_key]
+        return self._bindings[cache_key]
       except KeyError:
-        needs_new_binding = True
-
-    if needs_new_binding:
-      binding = Binding(weakref.proxy(self), namespace, named_index)
-      self._bindings[cache_key] = binding
+        pass
 
+    binding = Binding(weakref.proxy(self), namespace, named_index)
+    self._bindings[cache_key] = binding
     return binding