add option to '_normalize_matrix_blocks' for more informative errors

We allow the function that produced the data to be passed to
'_normalize_matrix_blocks', and any error messages are augmented
with this information. This may prove useful for debugging.

kwant/builder.py

@@ -2127,7 +2127,8 @@ class _VectorizedFinalizedBuilderMixin(_FinalizedBuilderMixin):
             to_family.norbs,
             from_family.norbs if not is_onsite else to_family.norbs,
         )
-        ham = system._normalize_matrix_blocks(ham, expected_shape)
+        ham = system._normalize_matrix_blocks(ham, expected_shape,
+                                              calling_function=val)
 
         return ham
 

kwant/operator.pyx

@@ -334,7 +334,8 @@ class _FunctionalOnsite:
             _raise_user_error(exc, self.onsite, vectorized=False)
 
         expected_shape = (len(site_offsets), norbs, norbs)
-        return _normalize_matrix_blocks(ret, expected_shape)
+        return _normalize_matrix_blocks(ret, expected_shape,
+                                        calling_function=self.onsite)
 
 
 class _VectorizedFunctionalOnsite:
@@ -353,7 +354,8 @@ class _VectorizedFunctionalOnsite:
             _raise_user_error(exc, self.onsite, vectorized=True)
 
         expected_shape = (len(sites), sites.family.norbs, sites.family.norbs)
-        return _normalize_matrix_blocks(ret, expected_shape)
+        return _normalize_matrix_blocks(ret, expected_shape,
+                                        calling_function=self.onsite)
 
 
 class _FunctionalOnsiteNoTransform:
@@ -371,7 +373,8 @@ class _FunctionalOnsiteNoTransform:
             _raise_user_error(exc, self.onsite, vectorized=False)
 
         expected_shape = (len(site_offsets), norbs, norbs)
-        return _normalize_matrix_blocks(ret, expected_shape)
+        return _normalize_matrix_blocks(ret, expected_shape,
+                                        calling_function=self.onsite)
 
 
 class _DictOnsite:

kwant/system.py

@@ -742,13 +742,16 @@ def is_vectorized(syst):
     return isinstance(syst, (FiniteVectorizedSystem, InfiniteVectorizedSystem))
 
 
-def _normalize_matrix_blocks(blocks, expected_shape):
+def _normalize_matrix_blocks(blocks, expected_shape, *, calling_function=None):
     """Normalize a sequence of matrices into a single 3D numpy array
 
     Parameters
     ----------
     blocks : sequence of complex array-like
     expected_shape : (int, int, int)
+    calling_function : callable (optional)
+        The function that produced 'blocks'. If provided, used to give
+        a more helpful error message if 'blocks' is not of the correct shape.
     """
     try:
         blocks = np.asarray(blocks, dtype=complex)
@@ -766,9 +769,11 @@ def _normalize_matrix_blocks(blocks, expected_shape):
     if blocks.shape != expected_shape:
         msg = (
             "Expected values of shape {}, but received values of shape {}"
-            .format(expected_shape, blocks.shape)
+                .format(expected_shape, blocks.shape),
+            "when evaluating {}".format(calling_function.__name__)
+                if callable(calling_function) else "",
         )
-        raise ValueError(msg)
+        raise ValueError(" ".join(msg))
 
     return blocks