improve error message in _normalize_matrix_blocks

kwant/system.py

@@ -759,17 +759,23 @@ def _normalize_matrix_blocks(blocks, expected_shape, *, calling_function=None):
         raise ValueError(
             "Matrix elements declared with incompatible shapes."
         ) from None
+    original_shape = blocks.shape
+    was_broadcast = True  # Did the shape get broadcasted to a more general one?
     if len(blocks.shape) == 0:  # scalar → broadcast to vector of 1x1 matrices
         blocks = np.tile(blocks, (expected_shape[0], 1, 1))
     elif len(blocks.shape) == 1:  # vector → interpret as vector of 1x1 matrices
         blocks = blocks.reshape(-1, 1, 1)
     elif len(blocks.shape) == 2:  # matrix → broadcast to vector of matrices
         blocks = np.tile(blocks, (expected_shape[0], 1, 1))
+    else:
+        was_broadcast = False
 
     if blocks.shape != expected_shape:
         msg = (
             "Expected values of shape {}, but received values of shape {}"
                 .format(expected_shape, blocks.shape),
+            "(broadcasted from shape {})".format(original_shape)
+                if was_broadcast else "",
             "when evaluating {}".format(calling_function.__name__)
                 if callable(calling_function) else "",
         )