code: def context_range(self, context):
          
          if not context.startswith(self.prefix):
              context = self.prefix + '.' + context
  
          lo = hi = None
          for idx, line_context in enumerate(self.lines, 1):
  
              # context is hierarchical -- context spans itself
              # and any suffix.
              if line_context.startswith(context):
                  lo = lo or idx
                  hi = idx
  
          if lo is None:
              raise ValueError("Context %s does not exist in file %s" %
                               (context, self.filename))
  
          return lo, hi + 1
docs: Return the 1-offset, right-open range of lines spanned by
          a particular context name.
  
          Parameters
          ----------
          context : str
  
          Raises
          ------
          ValueError, if context is not present in the file.