Book Store in Python
To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiplebook purchases.
1  exercism fetch python bookstore

Book Store
To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts on multiple book purchases.
One copy of any of the five books costs $8.
If, however, you buy two different books, you get a 5% discount on those two books.
If you buy 3 different books, you get a 10% discount.
If you buy 4 different books, you get a 20% discount.
If you buy all 5, you get a 25% discount.
Note: that if you buy four books, of which 3 are different titles, you get a 10% discount on the 3 that form part of a set, but the fourth book still costs $8.
Your mission is to write a piece of code to calculate the price of any conceivable shopping basket (containing only books of the same series), giving as big a discount as possible.
For example, how much does this basket of books cost?
 2 copies of the first book
 2 copies of the second book
 2 copies of the third book
 1 copy of the fourth book
 1 copy of the fifth book
One way of grouping these 8 books is:
 1 group of 5 > 25% discount (1st,2nd,3rd,4th,5th)
 +1 group of 3 > 10% discount (1st,2nd,3rd)
This would give a total of:
 5 books at a 25% discount
 +3 books at a 10% discount
Resulting in:
 5 x (8  2.00) == 5 x 6.00 == $30.00
 +3 x (8  0.80) == 3 x 7.20 == $21.60
For a total of $51.60
However, a different way to group these 8 books is:
 1 group of 4 books > 20% discount (1st,2nd,3rd,4th)
 +1 group of 4 books > 20% discount (1st,2nd,3rd,5th)
This would give a total of:
 4 books at a 20% discount
 +4 books at a 20% discount
Resulting in:
 4 x (8  1.60) == 4 x 6.40 == $25.60
 +4 x (8  1.60) == 4 x 6.40 == $25.60
For a total of $51.20
And $51.20 is the price with the biggest discount.
Exception messages
Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.
To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
raise Exception
, you should write:
1 
raise Exception("Meaningful message indicating the source of the error")

Running the tests
To run the tests, run the appropriate command below (why they are different):
 Python 2.7:
py.test book_store_test.py
 Python 3.4+:
pytest book_store_test.py
Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
python m pytest book_store_test.py
Common pytest
options

v
: enable verbose output 
x
: stop running tests on first failure 
ff
: run failures from previous test before running other test cases
For other options, see python m pytest h
Submitting Exercises
Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/bookstore
directory.
You can find your Exercism workspace by running exercism debug
and looking for the line that starts with Workspace
.
For more detailed information about running tests, code style and linting, please see the help page.
Source
Inspired by the harry potter kata from CyberDojo. http://cyberdojo.org
Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.