input: 
  def sort_by_attr(objects, attr):
      """ Sort a list of objects by a specified attribute.
      >>> sort_by_attr([{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "age")
      [{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}]
      >>> sort_by_attr([{"name": "Charlie", "age": 18}, {"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "name")
      [{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}, {"name": "Charlie", "age": 18}]
      """
      return sorted(objects, key=lambda item: item[attr])
  
output:     return sorted(objects, key=lambda item: item[attr])