20.9. Sorting Lists of Instances

You previously learned how to sort lists. Sorting lists of instances of a class is fundamentally the same as sorting lists of objects of any other type. There are two main ways to sort lists of instances: (1) by providing a key function as a parameter to sorted() (or .sort()) or by (2) defining a “comparison operator” that determines how two instances should be compared (specifically, given two instances, which one should come first). We will describe both ways here.

20.9.1. Approach 1: Sorting Lists of Instances with key

Previously, you have seen how to provide such a function as input when sorting lists of other kinds of objects. For example, given a list of strings, you can sort them in ascending order of their lengths by passing len as the key parameter. Note that if you refer to a function by name, you give the name of the function without parentheses after it, because you want the function object itself. The sorted function will take care of calling the function, passing the current item in the list. Thus, in the example below, we write key=len and not key=len().

When each of the items in a list is an instance of a class, the function you pass for the key parameter takes one instance as an input and returns a number. The instances will be sorted by their returned numbers.

Sometimes you will find it convenient to define a method for the class that does some computation on the data in an instance. In this case, our class is too simple to really illustrate that. But to simulate it, I’ve defined a method sort_priority that just returns the price that’s stored in the instance. Now, that method, sort_priority takes one instance as input and returns a number. So it is exactly the kind of function we need to provide as the key parameter for sorted. Here it can get a little confusing: to refer to that method, without actually invoking it, you can refer to Fruit.sort_priority. This is analogous to the code above that referred to len rather than invoking len().

20.9.2. Approach 2: Defining Sort Orders with Comparison Operators

Another approach to sorting lists of instances is to specify a “comparison operator” for the class—a method that takes two instances as arguments and “decides” which should come first. One advantage of this approach is that you can call sorted on a list of instances without specifying a value for key and it will sort in the order you defined.

To do this, we can define a method named __lt__ which stands for “less than”. Note that this method starts and ends with two underscores. This signifies that it is a special method, just like __init__ and __str__. Our method, __lt__, takes two instances as arguments: self and an argument for another instance. It returns True if the self instance should come before the other instance, and False otherwise. Normally, __lt__ is called when we try to use the less than operator (<) on class instances; Python translates the expression a < b into a.__lt__(b). However, we can also use __lt__ to decide which of two instances should come first in a sorted list. For example, if we wanted to sort instances of Fruit by prices by default, we could define __lt__ as follows:

When we call sorted(L) without specifying a value for the key parameter, it will sort the items in the list using the __lt__ method defined for the class of items. sorted() will automatically call the __lt__ method, passing in two instances from the list. Calling __lt__ when self is Fruit("Apple", 10) and other is Fruit("Cherry", 5) returns False (because the price of the apple is not less than the price of the cherry) so this means Cherry should come before Apple in the sorted list.

If we wanted to sort by names, we could define __lt__ differently. Note that when we call ``<`` on strings, it does an alphabetical comparison; ``”Apple” < “Cherry”`` is ``True``. We can take advantage of this in our ``__lt__`` method:

Finally, note that if we pass in a function for the key parameter when we call sorted() (approach 1), it will use that key function instead of calling the __lt__ method. You can try putting a print statement inside the __lt__ method to see this for yourself: __lt__ will not be called when you provide a key function but it will be called when you don’t provide a key function.

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