code: def boundary(self):
          
          i = self._boundary
          for name, _i in BOUNDARIES.items():
              if i==_i:
                  return name
          return i
docs: Get or set the boundary module.
  
          Available boundary modules are:
  
          - ``'none'`` (default)
          - ``'open'``
          - ``'periodic'``
          - ``'shear'``
          
          Check the online documentation for a full description of each of the modules.