Working in Python on Jupyter#
This is an introduction to Jupyter notebooks for people using Working in Python, a high school course adapted from the third edition of Think Python by Allen B. Downey.
A Jupyter notebook is a document that contains text, code, and results from running the code. You can read a notebook like a book, but you can also run the code, modify it, and develop new programs.
Jupyter notebooks run in a web browser, so you can run them without installing any new software. But they have to connect to a Jupyter server somewhere.
On this site, that part is already taken care of: every chapter opens as a ready-to-run notebook directly in your browser, with nothing to install and no account required.
On the starting page for the book you will find a link for each chapter. Most chapters also offer a Google Colab link as an alternative, if you’d rather work there instead.
Ignore this cell — used when running JupyterLite.
What is a notebook?#
A Jupyter notebook is made up of cells, where each cell contains either text or code. This cell contains text.
The following cell contains code.
Two kinds of cells#
Every cell in a notebook is one of two types:
A markdown cell holds formatted text — headings, paragraphs, lists, links. This cell is a markdown cell.
A code cell holds Python code that Jupyter runs when you press “Run”.
You can tell which kind a cell is by what happens when you run it: a markdown cell re-renders as formatted text, while a code cell executes and (usually) prints a result underneath.
print('Hello')
Hello
Click on the previous cell to select it. You should see a button on the left with a triangle inside a circle, which is the icon for “Play”. If you press this button, Jupyter runs the code in the cell and displays the result.
When you run code in a notebook for the first time, it might take a few seconds to start. And if it’s a notebook you didn’t write, you might get a warning message. If you are running a notebook from a source you trust, which I hope includes me, you can press “Run Anyway”.
Instead of clicking the “Play” button, you can also run the code in a cell by holding down Shift and pressing Enter.
Most Jupyter interfaces, including the one on this site, also let you add a new cell below the one you’re on — usually with a “+” button in the toolbar. Try it: select this cell, then add a new cell below it.
A quick look at Markdown#
Markdown formats text using plain characters instead of buttons. Some of the most common markup:
You type |
You get |
|---|---|
|
a large heading |
|
a smaller heading |
|
bold |
|
italic |
|
|
|
a link |
|
a bulleted list item |
Double-click any markdown cell in this notebook — including this one — to see its raw Markdown source. Press Shift+Enter to render it back to formatted text.
Add some text to the cell.
You can use the buttons to format it, or you can mark up the text using Markdown.
When you are done, hold down Shift and press Enter, which will format the text you just typed and then move to the next cell.
At any time Jupyter is in one of two modes:
In command mode, you can perform operations that affect cells, like adding and removing entire cells.
In edit mode, you can edit the contents of a cell.
With text cells, it is obvious which mode you are in. In edit mode, the cell is split vertically, with the text you are editing on the left and the formatted text on the right. And you’ll see text editing tools across the top. In command mode, you see only the formatted text.
With code cells, the difference is more subtle, but if there’s a cursor in the cell, you are in edit mode.
To go from edit mode to command mode, press ESC.
To go from command mode to edit mode, press Enter.
When you are done working on a notebook, you can close the window, but any changes you made will disappear. If you make any changes you want to keep, open the File menu in the upper left. You’ll see several ways you can save the notebook.
If you have a Google account, you can save the notebook in your Drive.
If you have a GitHub account, you can save it on GitHub.
Or if you want to save the notebook on your computer, select “Download” and then “Download .ipynb” The suffix “.ipynb” indicates that it is a notebook file, as opposed to “.py”, which indicates a file that contains Python code only.
Code for Think Python#
At the beginning of each notebook, you’ll see a cell with code like this:
from os.path import basename, exists
def download(url):
filename = basename(url)
if not exists(filename):
from urllib.request import urlretrieve
local, _ = urlretrieve(url, filename)
print("Downloaded " + str(local))
return filename
download('https://raw.githubusercontent.com/porttack/working-in-python/v3/working_in_python.py')
import working_in_python
You don’t need to know how this code works, but when you get to the end of the book, most of it will make sense.
As you might guess, it downloads a file – specifically, it downloads working_in_python.py, which contains Python code provided specifically for this book.
The last line “imports” this code, which means we can use the code in the notebook.
In other chapters, you will see code that downloads diagram.py, which is used to generated the diagrams in the book, and jupyturtle.py, which is used in several chapters to create turtle graphics.
In some places you will see a cell like this that begins with %%expect.
abs 42
%%expect is not part of Python – it is a Jupyter “magic command” that indicates that we expect the cell to product an error.
When you see this command, it means that the error is deliberate, usually intended to warn you about a common pitfall.
Or, if you are ready to get started, click here to read Chapter 1.
Working in Python — modified by Eric Brown for a high school Computer Science Principles class. Source and modifications: github.com/porttack/working-in-python
Think Python, 3rd edition.
Copyright 2023 Allen B. Downey
License: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International