Python Polars: The Definitive Guide

Chapter 8. Continuing Expressions

In the previous chapter you learned how to begin an expression from scratch. A bare expression only gets you so far. In this chapter, you’ll learn how to continue an expression by adding additional operations (or methods).

More specifically, you’ll learn how to:

• Perform mathematical transformations

• Work with missing values

• Apply smoothing to values

• Select specific values

• Summarize values using statistics

A Plethora of Methods

There are more than 138 methods discussed in this chapter. It’s not possible to explain and demonstrate every single method in full detail. Please refer to the Polars API Reference for more details and examples.

For some code snippets in this chapter, we use the math and numpy modules for accessing certain constants, such as math.pi, and for generating random values:

import math
import numpy as np

print(f"{math.pi=}")
rng = np.random.default_rng(1729)
print(f"{rng.random()=}")

math.pi=3.141592653589793
rng.random()=0.03074202960516803

Let’s get into how this chapter is structured.

The instructions to get any files you might need are in Chapter 2. We assume that you have the files in the data subdirectory.

Types of Operations

Rather than presenting 138 methods as one long list, we’ve organized them into five sections according to which inputs they use and the shape of their output. Within those five sections we’ve grouped methods into categories when applicable. Methods that do not fall into any category are ...