Beginning Python - From Novice to Professional

CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS 181
Initialize the arithmetic sequence.
start - the first value in the sequence
step - the difference between two adjacent values
changed - a dictionary of values that have been modified by
the user
"""
self.start = start
# Store the start value
self.step = step
# Store the step value
self.changed = {}
# No items have been modified
def __getitem__(self, key):
"""
Get an item from the arithmetic sequence.
"""
checkIndex(key)
try: return self.changed[key]
except KeyError:
return self.start + key*self.step
# Modified?
# otherwise...
# ...calculate the value
def __setitem__(self, key, value):
"""
Change an item in the arithmetic sequence.
"""
checkIndex(key)
self.changed[key] = value
# Store the changed value
This implements an arithmetic sequence, a sequence of numbers in which each is greater
than the previous one by a constant amount. The first value is given by the constructor parameter
start (defaulting to zero), while the step between the values is given by step (defaulting to one).
You allow the user to change some of the elements by storing the exceptions to the general rule in
a dictionary called changed. If the element hasn’t been changed, it is calculated as start+key*step.
Here is an example of how you can use this class:
>>> s = ArithmeticSequence(1, 2)
>>> s[4]
9
>>> s[4] = 2
>>> s[4]
2
>>> s[5]
11
Note that it is illegal to delete items, which is why I haven’t implemented __del__: