Live Coding Session

🐍 Booleans & Conditionals, Asked of Data

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

Every question you will ask of a dataset this week starts as a yes-or-no question about one 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 learn to write those questions, to combine them, and to act on the answers. By the end of the hour 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 both start from an empty notebook and write everything together, live. There is no code on this page to copy, on purpose.

  • I write a line, explain it, and run it. You write the same line and run it.
  • When something breaks, we fix it together. Things will break. That’s the useful part.
  • If you fall behind, say so. Someone else is behind too, and I would much rather slow down than lose you.
  • Type the code. Do not copy it from a neighbor’s screen. The typing is the point.

Set up your notebook

  1. Create a new notebook named Session_3A_Booleans_and_Conditionals.ipynb.

  2. Add a title cell:

# Day 3: Session 3A - Booleans & Conditionals

Date: 09/02/2026
  1. Add the following markdown cells now, in this order, leaving space under each. We’ll fill in the code as we go, and having the structure in place means your notes stay organized instead of becoming 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
  1. Under each heading, add an empty code cell.

That’s your scaffold. Now close this page and switch to your notebook.

The data

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

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

You already know what’s in it, so nothing today will be a surprise about the data. That 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. That line is the filter sentence, and it is the single most useful thing you will learn this week. Everything in this session exists to make that line make sense.

Resources