Higher-order functions

We can achieve expressive, succinct programs using higher-order functions. These are functions that accept a function as an argument or return a function as a value. We can use higher-order functions as a way to create composite functions from simpler functions.

Consider the Python max() function. We can provide a function as an argument and modify how the max() function behaves.

Here's some data we might want to process:

>>> year_cheese = [(2000, 29.87), (2001, 30.12), (2002, 30.6), (2003, 
30.66),(2004, 31.33), (2005, 32.62), (2006, 32.73), (2007, 33.5), 
(2008, 32.84), (2009, 33.02), (2010, 32.92)]

We can apply the max() function, as follows:

>>> max(year_cheese)
(2010, 32.92)

The default behavior is to simply compare each tuple in the sequence. This will return the tuple with the largest value on position 0.

Since the max() function is a higher-order function, we can provide another function as an argument. In this case, we'll use lambda as the function; this is used by the max() function, as follows:

>>> max(year_cheese, key=lambda yc: yc[1])
(2007, 33.5)  

In this example, the max() function applies the supplied lambda and returns the tuple with the largest value in position 1.

Python provides a rich collection of higher-order functions. We'll see examples of each of Python's higher-order functions in later chapters, primarily in Chapter 5, Higher-Order Functions. We'll also see how we can easily write our own higher-order functions.