Warn about missing IndexMap in Topology (#3853)

* Add warning

* Adjust Python layer

Author: chrisrichardson

cpp/dolfinx/mesh/Topology.cpp

@@ -825,7 +825,14 @@ std::shared_ptr<const common::IndexMap> Topology::index_map(int dim) const
   if (_entity_types[dim].size() > 1)
     throw std::runtime_error(
         "Multiple index maps of this dimension. Call index_maps instead.");
-  return this->index_maps(dim).at(0);
+  auto im = index_maps(dim);
+  if (im.empty())
+  {
+    throw std::runtime_error(std::format(
+        "Missing IndexMap in Topology. Maybe you need to create_entities({}).",
+        dim));
+  }
+  return im.at(0);
 }
 //-----------------------------------------------------------------------------
 std::shared_ptr<const graph::AdjacencyList<std::int32_t>>

python/dolfinx/mesh.py

@@ -183,13 +183,7 @@ def index_map(self, dim: int) -> _cpp.common.IndexMap:
         Returns:
             Index map for the entities of dimension ``dim``.
         """
-        if (imap := self._cpp_object.index_map(dim)) is not None:
-            return imap
-        else:
-            raise RuntimeError(
-                f"Entities of dimension {dim} has not been computed."
-                f"Call `dolfinx.mesh.Topology.create_entities({dim}) first."
-            )
+        return self._cpp_object.index_map(dim)
 
     def index_maps(self, dim: int) -> list[_cpp.common.IndexMap]:
         """Get the IndexMaps that describes the parallel distribution of
@@ -200,14 +194,9 @@ def index_maps(self, dim: int) -> list[_cpp.common.IndexMap]:
 
         Returns:
             List of IndexMaps for the entities of dimension ``dim``.
+            May be empty if not yet computed.
         """
-        if (imaps := self._cpp_object.index_maps(dim)) is not None:
-            return imaps
-        else:
-            raise RuntimeError(
-                f"Entities of dimension {dim} have not been computed."
-                f"Call `dolfinx.mesh.Topology.create_entities({dim}) first."
-            )
+        return self._cpp_object.index_maps(dim)
 
     def interprocess_facets(self) -> npt.NDArray[np.int32]:
         """List of inter-process facets, if facet topology has been