Optional key parameter¶

If you want to sort things in some order other than the “natural” or or its reverse, you can provide an additional parameter, the key parameter. For example, suppose you want to sort a list of numbers based on their absolute value, so that -4 comes after 3? Or suppose you have a dictionary with strings as the keys and numbers as the values. Instead of sorting them in alphabetic order based on the keys, you might like to sort them in order based on their values.

First, let’s see an example, and then we’ll dive into how it works.

First, let’s define a function absolute that takes a number and returns its absolute value. (Actually, python provides a built-in function abs that does this, but we are going to define our own, for reasons that will be explained in a minute.)

Now, we can pass the absolute function to L1 in order to specify that we want the items sorted in order of their absolute value, rather than in order of their actual value.

What’s really going on there? We’ve done something pretty strange. Before, all the values we have passed as parameters have been pretty easy to understand: numbers, strings, lists, Booleans, dictionaries. Here we have passed a function object: absolute is a variable name whose value is the function. When we pass that function object, it is not automatically invoked. Instead, it is just bound to the formal parameter key of the function sorted.

We are not going to look at the source code for the built-in function sorted. But if we did, we would find somewhere in its code a parameter named key with a default value of None. When a value is provided for that parameter in an invocation of the function sorted, it has to be a function. What the sorted function does is call that key function once for each item in the list that’s getting sorted. It associates the result returned by that function (the absolute function in our case) with the original value. Think of those associated values as being little post-it notes that decorate the original values. The value 4 has a post-it note that says 4 on it, but the value -2 has a post-it note that says 2 on it. Then the sorted function rearranges the original items in order of the values written on their associated post-it notes.

To illustrate that the absolute function is invoked once on each item, during the execution of sorted, I have added some print statements into the code.

Note that this code never explicitly calls the absolute function at all. It passes the absolute function as a parameter value to the sorted function. Inside the sorted function, whose code we haven’t seen, that function gets invoked.