Live Coding Session

An enormously complex tangle of piping and valves with numerous loops and on/off spigots that control the flow. MidJourney 5

Almost every question we ask of a dataset this week starts as a yes-or-no question about a single value. Was this day above freezing? Is this reading from the ozone monitor? Did this park get more than a million visitors?

Python has a type for the answer to a question like that, and it is called a boolean: a value that is either True or False. This morning we will learn how to write those questions, how to combine them, and what to do with the answers. By the end of the session you will have asked a question of an entire column at once, which is the idea the rest of today is built on.

How a live coding session works

We all start from an empty notebook and write everything together, live. There is no code on this page to copy, on purpose.

Kelly writes a line, explains it, and runs it. You write the same line and run it.
When something breaks, we fix it together. Things will break, and the fixing is the useful part.
If you fall behind, say so. Cella and Kelly would both much rather slow down than lose you.
Type the code yourself rather than copying it from a neighbor’s screen. Typing a line out, and then fixing it when it does not run, is how the syntax gets into your fingers.

Set up your notebook

Create a new notebook named Session_3A_Booleans_and_Conditionals.ipynb.
Add a title cell:

# Day 3: Session 3A - Booleans & Conditionals

Date: 09/02/2026

Add the following markdown cells now, in this order, leaving space under each. We will fill in the code as we go, and having the structure in place should keep your notes organized instead of turning them into one long scroll.

## 1. A yes-or-no question

## 2. The comparison operators

## 3. Asking two questions at once

## 4. if / elif / else

## 5. Asking a whole column

## 6. Counting the Trues

## 7. Where this goes next

Under each heading, add an empty code cell.

Your seven headings are the scaffold for the session. Read the next two sections before you switch to your notebook, because the file we use is named there.

The data

We’ll use the Toolik Field Station weather record, the same file you wrote a biography of yesterday:

https://eds-217-essential-python.github.io/data/toolik_weather.csv

You explored the file yesterday, so nothing this morning should be a surprise about the data, which leaves your attention free for the questions.

What we’ll cover

Booleans and comparisons (about 8 minutes)

True and False, and what type() says about them
the six comparison operators, including the difference between = and ==
reading one value out of a DataFrame with .loc[]
combining questions with and, or, and not

Conditionals (about 8 minutes)

if, and why the colon and the indentation are not optional
else for the leftover case
elif for sorting a value into one of several categories
writing a small classifier: cold day, cool day, warm day

Asking a whole column (about 9 minutes)

what happens when you compare an entire column to a number
the boolean mask, a column of True and False with the row labels still attached
&, |, and ~, which are what pandas uses in place of and, or, and not
why every piece of a combined mask needs its own parentheses
counting with .sum(), because True counts as 1

What you should be able to do afterwards

write a comparison and predict whether it gives True or False
write an if/elif/else block that puts one number into one of three categories
compare a column to a value and explain what you get back
combine two column comparisons with & and get the parentheses right the first time
say, in one sentence, what a boolean mask is for

Where this goes next

In the very next session you’ll put a mask inside square brackets and get back a smaller table. The line you write there is the filter pattern, and the booleans, the operators and the masks we practice this morning are the pieces it is made of.