input: Write a function to create a class in Python which satisfies the criteria given. The class should be named Song and should have two instance variables, title and artist. The class should also have three methods: str(), which returns a formatted string containing the title and artist; eq(), which checks if two instances of the class are equal based on their title and artist; and length(), which returns the length of the title string.
  
  class Song:
      def __init__(self, title, artist):
          self.title = title
          self.artist = artist
    
      def __str__(self):
          return f'Title: {self.title}\nArtist: {self.artist}'
      
      def __eq__(self, other):
          return self.artist == other.artist and self.title == other.title
  
      def length(self):
          return len(self.title)
  
  assert str(Song('Shape of You', 'Ed Sheeran')) == 'Title: Shape of You\nArtist: Ed Sheeran'
  assert Song('Shape of You', 'Ed Sheeran') == Song('Shape of You', 'Ed Sheeran')
  assert Song('Shape of You', 'Ed Sheeran') != Song('Bohemian Rhapsody', 'Queen')
  assert Song('Shape of You', 'Ed Sheeran').length() == 12
output: class Song:
      def __init__(self, title, artist):
          self.title = title
          self.artist = artist
    
      def __str__(self):
          return f'Title: {self.title}\nArtist: {self.artist}'
      
      def __eq__(self, other):
          return self.artist == other.artist and self.title == other.title
  
      def length(self):
          return len(self.title)