input: 
  def num_common_chars(str1: str, str2: str) -> int:
      """ Find the total number of common characters present in both strings.
      >>> num_common_chars("hello", "world")
      3
      >>> num_common_chars("abc", "def")
      0
      >>> num_common_chars("aaa", "a")
      1
      """
  
output:     common_chars = set(str1) & set(str2)
      return len(common_chars)