str.lower() -> str

#!/usr/bin/python3
str = "ABCDEFG"
print( str.lower() )

#实现lower
def my_lower(string):
  if not string:
    return None
  
  lst = list(string)
  for index,item in enumerate(lst):#同时列出数字和下标
    ascii_index = ord(item)
    if 65 <= ascii_index <= 90:
      s = chr(ascii_index+32)
      lst[index] = s
  return ''.join(lst)

if __name__ == '__main__':
  print(my_lower("2345ZRdV"))