Python in Excel Step-by-Step

Chapter 5Functions

Many Excel users spend a lot of time writing formulas. More often than not, these formulas call Excel functions like these:

• IF()
• SUM()
• AVERAGE()
• COUNT()
• MIN()
• MAX()

While most Excel users don’t do this, Excel has supported creating custom functions for many years using Visual Basic for Applications (VBA) code.

However, being able to create custom functions is so handy that Microsoft added the new LAMDBA() function in Excel to allow writing custom functions without VBA code.

Just as it’s handy to be able to write your own functions in Excel, so it is with Python.

In this chapter you’ll learn how to write your own Python functions.

5.1 INTRODUCING FUNCTIONS

Imagine that you have a column of sales numbers in an Excel worksheet and your manager has asked you to provide a summary of what’s going on with sales.

The first thing you might do is use Excel functions like SUM(), AVERAGE(), MIN(), and MAX() to get an initial understanding of the sales data.

As an Excel user, you’re undoubtedly highly experienced with using functions. You’ve also been building experience using Python functions like print(), type(), and len() from previous chapters.

This knowledge makes learning how to write your own Python functions much easier.

5.1.1 Defining Functions

You tell Python that you’re defining a function by using the def keyword. In addition to the def keyword, all Python functions must have the following:

• A name.
• A parameter list (which can be empty) contained with ...