Always set the bounding box on the forward transform.

gwcs/wcs.py

@@ -258,15 +258,20 @@ def set_transform(self, from_frame, to_frame, transform):
     def forward_transform(self):
         """
         Return the total forward transform - from input to output coordinate frame.
-
         """
-
-        if self._pipeline:
-            #return functools.reduce(lambda x, y: x | y, [step[1] for step in self._pipeline[: -1]])
-            return functools.reduce(lambda x, y: x | y, [step.transform for step in self._pipeline[:-1]])
-        else:
+        if not self._pipeline:
             return None
 
+        transform = functools.reduce(lambda x, y: x | y, [step.transform for step in self._pipeline[:-1]])
+
+        if self.bounding_box is not None:
+            # Currently compound models do not attempt to combine individual model
+            # bounding boxes. Get the forward transform and assign the bounding_box to it
+            # before evaluating it. The order Model.bounding_box is reversed.
+            transform.bounding_box = self.bounding_box
+
+        return transform
+
     @property
     def backward_transform(self):
         """
@@ -354,12 +359,6 @@ def __call__(self, *args, **kwargs):
         if 'fill_value' not in kwargs:
             kwargs['fill_value'] = np.nan
 
-        if self.bounding_box is not None:
-            # Currently compound models do not attempt to combine individual model
-            # bounding boxes. Get the forward transform and assign the bounding_box to it
-            # before evaluating it. The order Model.bounding_box is reversed.
-            transform.bounding_box = self.bounding_box
-
         result = transform(*args, **kwargs)
 
         if with_units:
@@ -1287,6 +1286,10 @@ def bounding_box(self):
 
         frames = self.available_frames
         transform_0 = self.get_transform(frames[0], frames[1])
+
+        if transform_0 is None:
+            return None
+
         try:
             bb = transform_0.bounding_box
         except NotImplementedError: