input: Write a function that takes a list of objects and returns a list of those objects sorted by one of their attributes. Provide the function name, arguments, and the expected output for given input.
  
  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": "Alice", "age": 21}, {"name": "Bob", "age": 20}], "name") == [{"name": "Alice", "age": 21}, {"name": "Bob", "age": 20}]
  assert sort_by_attr([{"name": "Charlie", "age": 19}], "age") == [{"name": "Charlie", "age": 19}]
  assert sort_by_attr([], "age") == []
output: def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])