def percent_diff(val1, val2):
  """Returns what percentage a change val2 is from val1.

  For example, if val=10 and val2=15, returns '+50%'.

  Args:
    val1: Base number.
    val2: Number to compare against the base number.
  """
  diff = (val2 - val1) / val1 * 100
  return f"+{diff:.1f}%" if diff >= 0 else f"{diff:.1f}%"