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Why Use Functions?

Before we get into the details, let’s establish a clear picture of what functions are all about. Functions are a nearly universal program-structuring device. You may have come across them before in other languages, where they may have been called subroutines or procedures. As a brief introduction, functions serve two primary development roles:

Maximizing code reuse and minimizing redundancy

As in most programming languages, Python functions are the simplest way to package logic you may wish to use in more than one place and more than one time. Up until now, all the code we’ve been writing has run immediately. Functions allow us to group and generalize code to be used arbitrarily many times later. Because they allow us to code an operation in a single place and use it in many places, Python functions are the most basic factoring tool in the language: they allow us to reduce code redundancy in our programs, and thereby reduce maintenance effort.

Procedural decomposition

Functions also provide a tool for splitting systems into pieces that have well-defined roles. For instance, to make a pizza from scratch, you would start by mixing the dough, rolling it out, adding toppings, baking it, and so on. If you were programming a pizza-making robot, functions would help you divide the overall “make pizza” task into chunks—one function for each subtask in the process. It’s easier to implement the smaller tasks in isolation than it is to implement the entire process at once. In general, functions are about procedure—how to do something, rather than what you’re doing it to. We’ll see why this distinction matters in Part VI, when we start making new object with classes.

In this part of the book, we’ll explore the tools used to code functions in Python: function basics, scope rules, and argument passing, along with a few related concepts such as generators and functional tools. Because its importance begins to become more apparent at this level of coding, we’ll also revisit the notion of polymorphism introduced earlier in the book. As you’ll see, functions don’t imply much new syntax, but they do lead us to some bigger programming ideas.