input: Write a function in Python that takes a list of objects and returns a list of those objects sorted by one of their attributes.
  
  def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])
   
  assert sort_by_attr([{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "age")==[{'name': 'Alice', 'age': 20}, {'name': 'Bob', 'age': 21}]
  assert sort_by_attr([{"name": "Charlie", "age": 25}, {"name": "Alice", "age": 20}], "name")==[{'name': 'Alice', 'age': 20}, {'name': 'Charlie', 'age': 25}]
  assert sort_by_attr([], "age")==[]
output: def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])