When we sort a list of items, we need a basis on which to compare the items to see whether one is bigger than another. If it's a list of numbers, Python just compares the numeric values; if it's a list of strings, Python compares the strings alphabetically. But what if we're sorting a list of restaurants? As we saw earlier, the sort() method uses the first field of a Restaurant object (in our case, its name) for comparison. We call the basis of comparison the sort key. Wouldn't it be convenient if we could sort lists of complex objects based on some other sort key? [Note: nothing here in part (d) requires a for-loop or an if-statement.]

(d.1) Define a function called restaurant_price that takes one argument, a Restaurant, and returns the value of the price field of that Restaurant. This is quite short and easy. But note that whenever a lab problem asks you to define a function, you also need to include in your file some calls to that function that test it, calls that demonstrate that the function works correctly. So define a list containing a few Restaurants (maybe copy the list from last week's lab) and print out the results of calling restaurant_price on those Restaurants. (You may use assert statements, which are a way of partially automating this testing process.)

This is the list from last week's work.

from collections import namedtuple
Restaurant = namedtuple('Restaurant', 'name cuisine phone dish price')
# Restaurant attributes: name, kind of food served,
 phone number, best dish, price of that dish
RL = [
    Restaurant("Thai Dishes", "Thai", "334-4433", "Mee Krob", 12.50),
    Restaurant("Nobu", "Japanese", "335-4433", "Natto Temaki", 5.50),
    Restaurant("Nonna", "Italian", "355-4433", "Stracotto", 25.50),
    Restaurant("Jitlada", "Thai", "324-4433", "Paht Woon Sen", 15.50),
    Restaurant("Nola", "New Orleans", "336-4433", "Jambalaya", 5.50),
    Restaurant("Noma", "Modern Danish", "337-4433", "Birch Sap", 35.50),
    Restaurant("Addis Ababa", "Ethiopian", "337-4453", "Yesiga Tibs",
 10.50) ]

(d.2) Write a sequence of statements that prints out the list of Restaurants RC in order from least expensive to most expensive (best dish). [Hint: As an argument to the sort() method, saykey=restaurant_price. Take a second to read this again, paying attention to the terminology so you know what this is telling you to write in Python. Interpreting technical terminology is an important skill; it may also help if you look at the entry for sort() in help(list). Giving key=restaurant_price as an argument to sort() tells sort() how to get the value from each Restaurant that it will compare: By default (without our saying anything), it used the first field, the name of the Restaurant; the key= argument lets us specify a function that takes a Restaurant as input and returns a value-in our example, its price field-to use instead of the name when comparing Restaurants during the sorting process. What's tricky is that we supply the name of a function after key=, but as we'll see later, functions can be useful as components of other computations.]

(d.3) Write a function called costliest that takes a list of Restaurants as its argument and returns the name of the highest-priced restaurant on that list (based on the price of the restaurant's best dish, of course). [You need to pay close attention to the wording of technical specifications like this. This function returns a string, the name of the costliest restaurant; it doesn not return the whole Restaurant object.]

To test this function, create a short list of Restaurants and print the value of calling costliest with that list as its argument.

(d.4) You may have noticed that after calling your costliest function with a list of Restaurants, two things happen: costliest returns the name of the restaurant and the list has been reordered by price. Try it out: Print the list before calling costliest and then print it again afterwards; whatever its order beforehand, it's ordered by price after the call. This is a side effect: something a function does besides compute and return a value. Sometimes side effects are a required part of solving a problem: printing, for example, or drawing on a tkinter canvas. But when we're manipulating data in our program, generally it's best if a function does its work by returning a value and leaves everything else unchanged.