input: Write a function to calculate the distance between two points on a two-dimensional plane. The function should take four arguments: x1, y1 for the coordinates of the first point, and x2, y2 for the coordinates of the second point. The function should return the distance between these two points as a floating point number.
  
  assert distance_between_points(3, 4, -9, -2) == 13.0
  assert distance_between_points(0, 0, 1, 1) == 1.4142135623730951
  assert distance_between_points(-2, -3, -2, -3) == 0.0
  assert distance_between_points(1, 1, 4, 5) == 5.0
output: def distance_between_points(x1, y1, x2, y2):
      return ((x2 - x1) ** 2 + (y2 - y1) ** 2) ** 0.5