input: 
  def filter_vowels(string):
      """Filter out all the vowels from a given string.
      >>> filter_vowels('hello world')
      'hll wrld'
      >>> filter_vowels('aeiou')
      ''
      >>> filter_vowels('bcdfghjklmnpqrstvwxyz')
      'bcdfghjklmnpqrstvwxyz'
      """
      vowels = ('a', 'e', 'i', 'o', 'u')
      return ''.join(c for c in string if c.lower() not in vowels)
  
output:     vowels = ('a', 'e', 'i', 'o', 'u')
      return ''.join(c for c in string if c.lower() not in vowels)