Interactive Session

A cartoon depicting the idea of a variable. MidJourney 5

Now that you have Positron open and a notebook of your own, let’s meet the handful of Python building blocks you’ll use in just about every notebook this week: variables, strings, and the print() and type() functions. Then we’ll get to the star of today, f-strings, which build readable output out of the values you have computed. We’re keeping the vocabulary small on purpose, so that you leave today using all of it fluently rather than half-remembering a longer list.

Getting Started

Set up your notebook using the same ritual from this morning:

Create the file. In the Explorer, hover over the EDS217 heading and click New File…, then type the name in full, extension included: Session_1B_Python_Essentials.ipynb
Check the kernel. The Kernel Selector in the notebook’s action bar should read Python 3.11.15 (Conda: eds217). If it reads anything else, click it, choose Change Kernel, and pick the eds217 entry.
Add a title cell. Click + Markdown in the action bar, and give it this content, with today’s date:

# Day 1: Session 1B - Python Essentials

[Session Webpage](https://eds-217-essential-python.github.io/course-materials/interactive-sessions/1b_python_essentials.html)

Date: 08/31/2026

Save with Ctrl + S (Cmd + S on macOS), and keep saving as you go.

Add a code cell below the title cell to start writing Python.

Variables and assignment

A variable is a name that refers to a value. You create one with the assignment operator, =. The name goes on the left, the value on the right.

Code

temperature = 21
site_name = "Santa Barbara"

Nothing prints when you assign a variable, because Python stores the value away instead of displaying it. Ask for the name again and you get the value back:

Code

temperature

Values come in different types. The two numeric types we use today are integers (whole numbers, int) and floats (decimal numbers, float):

Code

count = 42          # an int
rainfall = 3.5      # a float

You can do basic arithmetic with numbers, just like a calculator, and store the result in a new variable:

Code

high_temp = 24
low_temp = 12
temp_range = high_temp - low_temp
temp_range

We’ll cover the comparison operators on Day 3. Today we only need basic +, -, * and /.

Note

🐍 Choose variable names that say what the value is (site_name, not s). Python style favors lowercase names with underscores. Names can’t start with a number, and Python keeps 35 words for itself, so class = 5 raises a SyntaxError when you run the cell.

✏️ Test your knowledge

Create two variables, morning_temp and afternoon_temp, give them values, and compute their difference into a new variable called daily_swing. Display daily_swing.

Strings

A string is a piece of text. You write one by wrapping characters in quotes, and single quotes and double quotes do the same job:

Code

greeting = 'hello'
place = "Bren Hall"

The only practical difference is which quote character you can then use inside the text without ending the string early. Use double quotes when your text contains an apostrophe:

Code

note = "It's a sunny day."

You can also write a multi-line string using triple quotes, which is handy for longer blocks of text:

Code

description = """This dataset records
daily air temperature
at three stations."""

Note

🐍 A string is just data. The quotes tell Python to treat the characters between them as text. We’ll learn ways to transform text (cleaning, splitting, reformatting) later in the course. For now we just need to create strings and print them.

✏️ Test your knowledge

Create a string variable favorite_place holding the name of somewhere you like. Then create a second string that includes an apostrophe (like "I'm from Ventura.") and make sure it doesn’t cause an error.

Knowing your types: `type()`

When you’re not sure what kind of value a variable holds, ask Python with the type() function:

Code

type(count)

int

Code

type(rainfall)

float

Code

type(site_name)

str

int, float, and str are the three types we use today. Knowing a value’s type tells you what you can do with it: you can subtract one number from another, but Python raises an error if you try to subtract one string from another.

✏️ Test your knowledge

Predict the type of each of these before you run it, then check by running print(type(19)), and the same for 19.0 and "19". Were any surprising?

Printing with `print()`

Asking for a variable’s value shows it only when it’s the last line of a cell. To display values whenever and wherever you want, use the print() function:

Code

print("Data loaded.")
print(site_name)

Data loaded.
Santa Barbara

print() can take several values at once, separating them with spaces:

Code

print("Site:", site_name, "-- readings:", count)

Site: Santa Barbara -- readings: 42

✏️ Test your knowledge

In a single cell, print two lines: one showing your favorite_place, and one showing the daily_swing you computed earlier. Use a separate print() for each.

f-strings: readable output

Stitching text and variables together with commas works, but it gets awkward fast. The modern, readable way to build a message from your data is the f-string (formatted string literal). We’ll spend the most time here.

The recipe

An f-string is a string with the letter f right before the opening quote. Inside it, anything you put in curly braces {} is replaced by the value of the variable you name there:

Code

name = "Alice"
print(f"Hello, {name}!")

Hello, Alice!

The recipe is f"text {variable} more text". The f turns it into an f-string, and each {variable} becomes its value.

Important

The pattern to memorize is f"text {variable}". The f must come before the quote, and the variable name goes inside the braces. Forget the f and you’ll just print the braces literally, with no error message to tell you what went wrong.

You can drop in as many variables as you like:

Code

site_name = "Goleta"
count = 42
print(f"We recorded {count} readings at {site_name}.")

We recorded 42 readings at Goleta.

Assigning to a name that already exists replaces what was there, so site_name holds Goleta from here on rather than Santa Barbara.

f-strings work anywhere a string does. You can build one and store it in a variable, then print it later:

Code

summary = f"Station {site_name} is ready."
print(summary)

Station Goleta is ready.

R vs Python

Coming from R? An f-string is like sprintf() or glue(), but the variables sit right inside the text where they’ll appear, with no separate list of arguments to line up.

Practice with f-strings

Let’s build up some fluency. Given these variables:

Code

station = "CNSI Roof Top"
readings = 30
warmest = 27

Each f-string below turns one of the variables above into a sentence:

Code

print(f"The station is {station}.")
print(f"It logged {readings} readings today.")
print(f"The warmest reading was {warmest} degrees.")

The station is CNSI Roof Top.
It logged 30 readings today.
The warmest reading was 27 degrees.

✏️ Test your knowledge

Change warmest in the cell that defines it, run that cell, then run the cell of f-strings again. The last sentence should update on its own, without you touching the f-string itself! Then write a new f-string that mentions both station and readings in the same sentence.

✏️ Test your knowledge

You can also put a computed value in a variable and drop it into an f-string. Create coolest = 14, compute spread = warmest - coolest, and print: f"The temperature spread was {spread} degrees."

Putting it together: write your data summary

Here is the pattern you will use all week: compute a few values, then describe them in plain language with f-strings.

Rainfall is a good quantity to summarize because it accumulates. Three days of rain add up to a three-day total, and dividing by three gives an average daily rate. Start from this small rainfall log and compute a couple of values using basic arithmetic:

Code

site = "Santa Barbara"
day1_rain_mm = 5
day2_rain_mm = 0
day3_rain_mm = 7

n_days = 3
total_rain_mm = day1_rain_mm + day2_rain_mm + day3_rain_mm   # daily rainfall adds up to a total
average_rain_mm = total_rain_mm / n_days

✏️ Your turn: write 2 or 3 f-string sentences

Using the variables above (and the values you computed), write two or three f-string print() statements that summarize the rainfall log in complete sentences. For example, your output might read something like:

“We tracked rainfall across 3 days at Santa Barbara.”
“In total, Santa Barbara received 12 mm of rain.”
“That’s an average of 4.0 mm per day.”

Write your own versions in a code cell. Each sentence should include at least one variable inside { }.

Key Points

A variable names a value. Create one with name = value. Nothing prints on assignment.
Today’s types are int (whole numbers), float (decimals), and str (text). Check any value with type().
Strings are text wrapped in single or double quotes. Use triple quotes for multi-line text.
print() displays values anywhere in a cell and accepts several values at once.
f-strings, written f"text {variable}", are the readable way to combine text and values, and the one thing you should be able to write on your own after today.

Resources

Course Cheatsheet: First Steps covers Python basics at a glance.
Course Cheatsheet: print() covers printing and f-strings.
Python Docs: Formatted string literals is the official f-string reference. The formatting options there are for later. Today we only need f"text {variable}".

End interactive session 1B

Getting Started

Variables and assignment

Strings

Knowing your types: type()

Printing with print()

f-strings: readable output

The recipe

Practice with f-strings

Putting it together: write your data summary

Key Points

Resources

Knowing your types: `type()`

Printing with `print()`