input: Write a function to improve the given function by using Python built-in functions. The original function takes in an array of numbers and returns the sum of all the numbers. The improved function should achieve the same result more efficiently using Python built-in functions.
  assert sum_array([1, 2, 3, 4, 5])==15
  assert sum_array([10, 20, 30])==60
  assert sum_array([-1, -2, 3])==0
  assert sum_array([])==0
  assert sum_array([0, 0, 0, 0])==0
output: def sum_array(arr):
      return sum(arr)