Core Python Programming (2nd Edition)

arguments in Section 11.5.2. One caveat is that default arguments should be immutable objects;
mutable objects like lists and dictionaries act like static data, maintaining their contents with each
method call.
Example 13.1 shows how we can use the default constructor behavior to help us calculate some sample
total room costs for lodging at hotels in some of America's large metropolitan areas.
Example 13.1. Using Default Arguments with Instantiation (hotel.py)
Class definition for a fictitious hotel room rate calculator. The __init__() constructor
method initializes several instance attributes. A calcTotal() method is used to determine
either a total daily room rate or the total room cost for an entire stay.
1 class HotelRoomCalc(object):
2 'Hotel room rate calculator'
3
4 def __init__(self, rt, sales=0.085, rm=0.1):
5 '''HotelRoomCalc default arguments:
6 sales tax == 8.5% and room tax == 10%'''
7 self.salesTax = sales
8 self.roomTax = rm
9 self.roomRate = rt
10
11 def calcTotal(self, days=1):
12 'Calculate total; default to daily rate'
13 daily = round((self.roomRate *
14 (1 + self.roomTax + self.salesTax)), 2)
15 return float(days) * daily
The main purpose of our code is to help someone figure out the daily hotel room rate, including any
state sales and room taxes. The default is for the general area around San Francisco, which has an
8.5% sales tax and a 10% room tax. The daily room rate has no default value, thus it is required for any
instance to be created.
The setup work is done after instantiation by __init__() in lines 4-8, and the other core part of our code
is the calcTotal() method, lines 10-14. The job of __init__() is to set the values needed to determine
the total base room rate of a hotel room (not counting room service, phone calls, or other incidental
items). calcTotal() is then used to either determine the total daily rate or the cost of an entire stay if
the number of days is provided. The round() built-in function is used to round the calculation to the
closest penny (two decimal places). Here is some sample usage of this class:
>>> sfo = HotelRoomCalc(299) # new instance
>>> sfo.calcTotal() # daily rate
354.32
>>> sfo.calcTotal(2) # 2-day rate
708.64
>>> sea = HotelRoomCalc(189, 0.086, 0.058) # new instance
>>> sea.calcTotal()
216.22
>>> sea.calcTotal(4)
864.88
>>> wasWkDay = HotelRoomCalc(169, 0.045, 0.02) # new instance