input: from typing import List
  
  
  def list_countries(countries: List[str]) -> str:
      """ Returns a string containing all the countries in the input list, each separated by a comma.
      >>> list_countries(['Germany', 'France', 'Japan'])
      'Germany, France, Japan'
      >>> list_countries([])
      ''
      >>> list_countries(['Singapore'])
      'Singapore'
      """
output:     return ', '.join(countries)