title: How can I handle the base case of this recursive function? My base case is causing an output of zero
body: <pre><code>''' Returns number of pennies if pennies are doubled num_days times'''
  def double_pennies(num_pennies, num_days):
      total_pennies = 0
  
      if num_days == 0:
          return total_pennies
  
      else:
          total_pennies = double_pennies((num_pennies * 2), (num_days - 1));
  
      return total_pennies
  
  ''' Program computes pennies if you have 1 penny today,'''
  ''' 2 pennies after one day, 4 after two days, and so on'''
  starting_pennies = 1
  user_days = 10
  
  print('Number of pennies after', user_days, 'days: ', end="")
  print(double_pennies(starting_pennies, user_days))
  </code></pre>