Correct me if I am wrong but I think this should be analogous to how it was origionally:
```
if mesh and not structured_coords and not structured_set:
```
This portion of control flow is to handle a case when a structured mesh instance or file is being provided by the user. The way you have it, this control flow would pick up the case when a user supplies:
```
m = Mesh(structured=True)
```
The user would then get the "Found no structured meshes in..." error message rather than the more appropriate  ```MeshError``` on line 645.