code: def _transform_variable_to_expression(expression, node, context):
      
      variable_name = expression.variable_name
      if not variable_name.startswith(u'$'):
          raise AssertionError(u'Unexpectedly received variable name {} that is not '
                               u'prefixed with "$"'.format(variable_name))
      return bindparam(variable_name[1:])
docs: Transform a Variable compiler expression into its SQLAlchemy expression representation.
  
      Args:
          expression: expression, Variable compiler expression.
          node: SqlNode, the SqlNode the expression applies to.
          context: CompilationContext, global compilation state and metadata.
  
      Returns:
          Expression, SQLAlchemy expression.