The built-in function eval() takes a string argument and a dictionary.  The second argument should allow any instance which defines __getitem__ as opposed to just dictionaries.

The following example creates a type error:
  eval, argument 2: expected dictionary,   instance found

class SpreadSheet:
    _cells = {}
    def __setitem__( self, key, formula ):
        self._cells[key] = formula
    def __getitem__( self, key ):
        return eval( self._cells[key], self )

ss = SpreadSheet()
ss['a1'] = '5'
ss['a2'] = 'a1*5'
ss['a2']