tweak numpy code

ortools/linear_solver/python/model_builder.py

@@ -692,8 +692,9 @@ def dot_variable_container(
 ) -> LinearExprT:
     """Implementation of np.dot for VariableContainer objects."""
     if len(container.shape) != 1:
-        raise TypeError(
-            'dot_variable_container only supports 1D variable containers')
+        raise ValueError(
+            'dot_variable_container only supports 1D variable containers (shape ='
+            f' {container.shape}')
     indices: npt.NDArray[np.int32] = container.variable_indices
     if np.isscalar(arg):
         return _WeightedSum(

ortools/linear_solver/samples/assignment_mb.py

@@ -30,8 +30,7 @@ def main():
         [45, 110, 95, 115],
         [50, 100, 90, 100],
     ])
-    num_workers = len(costs)
-    num_tasks = len(costs[0])
+    num_workers, num_tasks = costs.shape
     # [END data_model]
 
     # Solver