input: Write a function that takes two strings as input and returns the total number of common characters present in both strings.
  
  def num_common_chars(str1, str2):
      common = 0
      for c in str1:
          if c in str2:
              common += 1
      return common
  
  assert num_common_chars("hello", "world") == 3
  assert num_common_chars("abc", "def") == 0
  assert num_common_chars("aaa", "a") == 1
  assert num_common_chars("", "abc") == 0
  assert num_common_chars("abc", "") == 0
output: def num_common_chars(str1, str2):
      common = 0
      for c in str1:
          if c in str2:
              common += 1
      return common