input: from typing import List, Dict
  
  def sort_by_attr(objects: List[Dict], attr: str) -> List[Dict]:
      """ Sort a list of dictionaries by a specified attribute of the dictionaries.
      >>> sort_by_attr([{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "age")
      [{'name': 'Alice', 'age': 20}, {'name': 'Bob', 'age': 21}]
      """
output:     return sorted(objects, key=lambda x: x.get(attr))