code: def worse(src_amount, src_currency, target_amount_obtained, target_currency, date=None, valid_days_max=None):
      
      calculated = convert(src_amount, src_currency, target=target_currency, date=date, valid_days_max=valid_days_max)
      worse = calculated - target_amount_obtained
      worse_src = convert(worse, target_currency, target=src_currency, date=date, valid_days_max=valid_days_max)
      if src_amount:
          return worse_src / src_amount * 100.0, worse_src, worse
      elif not target_amount_obtained:
          return 0.0, worse_src, worse
      else:
          return float('inf') if (target_amount_obtained < 0) else float('-inf'), worse_src, worse
docs: will calculate a difference between the calculated target amount and the amount you give as src_amount
        if you will obtain target_amount_obtained instead
      valid_days_max: see rate()
      returns a tuple: (percent, difference_src_currency, difference_target_currency)