old_contents: 
  class Source:
  
      def __init__(self, name, mdb_collection):
          self.name = name
          self.mdb_collection = mdb_collection
  
      def download_data(self, symbols, params=None):
          print('%s - downloading %s' % (self.name, symbols))
          data = []
          for i in range(0, len(symbols), 10):
              data.extend(self._download_data(symbols[i:i+10], params))
          self.mdb_collection.insert_many(data)
          print('%s - done!' % self.name)
  
      def _download_data(self, symbols, params):
          pass
  
new_contents: 
  class Source:
  
      def __init__(self, name, mdb_collection):
          self.name = name
          self.mdb_collection = mdb_collection
  
      def download_data(self, symbols, params=None):
          print('%s - downloading %s' % (self.name, symbols))
          data = []
          for i in range(0, len(symbols), 20):
              data.extend(self._download_data(symbols[i:i+20], params))
          self.mdb_collection.insert_many(data)
          print('%s - done!' % self.name)
  
      def _download_data(self, symbols, params):
          pass
  
subject: Increase batch size to 20