MCS 260 Fall 2020
Emily Dumas
All of this is Python-specific. Some other languages use the term interface for a similar concept, though the details differ quite a bit.
In Python, a sequence is an ordered container supporting access to its items by 0-based index.
Things you can do with a sequence seq:
len(seq)seq[3]seq[3] = valfor item in seq:If you create a class with the following methods, it can be used as a mutable sequence:
__len__() — returns the length __getitem__(idx) — returns item at given index__setitem__(idx,val) — set item at given indexThese methods form the (mutable) sequence protocol.
| code | becomes |
|---|---|
obj[1] | obj.__getitem__(1) |
obj[1]=60 | obj.__setitem__(1,60) |
len(obj) | obj.__len__() |
for x in obj: | for i in range(len(obj)): |
A geometric sequence (or geometric progression) is a sequence of numbers where the ratio between neighboring terms is constant.
Infinite example: $1,2,4,8,16,32,64,\ldots$
Finite example: $5,15,45,135,405$
Non-example: $6,8,10,12,14$
Let's make a class FiniteGeometricSequence that will represent a finite geometric sequence.
We'll keep track of start, ratio, and length, but will only compute terms when requested.
Let's support item assignment with __setitem__.
Adopt these conventions:
startstart the same but adjusts ratioStill more can be found in the collections.abc module, which contains classes you can subclass when implementing the protocols.