input: 
  def filter_vowels(string):
      vowels = ('a', 'e', 'i', 'o', 'u')
      return ''.join(c for c in string if c not in vowels)
  
  """
  >>> filter_vowels("hello world")
  'hll wrld'
  >>> filter_vowels("python")
  'pythn'
  """
  
output:     return ''.join(c for c in string if c.lower() not in vowels)