input: Write a function to find and return the maximum number from a given list of integers. If the list is empty, return None.
  
  def max_num(nums):
      if not nums:
          return None
      max = nums[0]
      for num in nums[1:]:
          if num > max:
              max = num
      return max
  
  assert max_num([5, 4, 2, 7, 12])==12
  assert max_num([-3, -5, -1, -2])==-1
  assert max_num([10])==10
  assert max_num([])==None
output: def max_num(nums):
      if not nums:
          return None
      max = nums[0]
      for num in nums[1:]:
          if num > max:
              max = num
      return max