input: Write a function to find the smallest number in a list of numbers. The function should return the smallest number in the list. If the list is empty, the function should return None.
  def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest
  
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([3, 7, 2, 8, 9])==2
  assert find_smallest_number([])==None
  assert find_smallest_number([10, 20, 30, 40, 50])==10
  assert find_smallest_number([1])==1
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest