Running and Quitting

Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How can I run Python programs?

Objectives

• Launch the Jupyter Notebook, create new notebooks, and exit the Notebook.

• Create Markdown cells in a notebook.

• Create and run Python cells in a notebook.

Getting the Data

The data we will be using is taken from the [gapminder][gapminder] dataset. Create a directory called python-novice-gapminder, and download the zip file python-novice-gapminder-data.zip into it. Then unzip the file (it should create a data folder. Then start a Jupyter notebook from inside this directory (see the second header below).

Python programs are plain text files.

• They have the .py extension to let everyone (including the operating system) know it is a Python program.
  • This is convention, not a requirement.
• It’s common to write them using a text editor but we are going to use the Jupyter Notebook.
• The bit of extra setup is well worth it because the Notebook provides code completion and other helpful features.
• Notebook files have the extension .ipynb to distinguish them from plain-text Python programs.
  • Can export as “pure Python” to run from the command line.

Use the Jupyter Notebook for editing and running Python.

• The Anaconda package manager is an automated way to install the Jupyter notebook.
  • See the setup instructions for Anaconda installation instructions.
• It also installs all the extra libraries it needs to run.

• Once you have installed Python and the Jupyter Notebook requirements, open a shell and type:

  $ jupyter notebook
  

• This will start a Jupyter Notebook server and open your default web browser.
• The server runs locally on your machine only and does not use an internet connection.
• The server sends messages to your browser.
• The server does the work and the web browser renders the notebook.
• You can type code into the browser and see the result when the web page talks to the server.
• This has several advantages:
  • You can easily type, edit, and copy and paste blocks of code.
  • Tab complete allows you to easily access the names of things you are using and learn more about them.
  • It allows you to annotate your code with links, different sized text, bullets, etc. to make it more accessible to you and your collaborators.
  • It allows you to display figures next to the code that produces them to tell a complete story of the analysis.

Screenshot of a Jupyter Notebook on quantum mechanics by Robert Johansson

How It’s Stored

• The notebook file is stored in a format called JSON.
• Just like a webpage, what’s saved looks different from what you see in your browser.
• But this format allows Jupyter to mix software (in several languages) with documentation and graphics, all in one file.

Use the keyboard and mouse to select and edit cells.

• Pressing the “return” key turns the border green and engages edit mode, which allows you to type within the cell.
• Because we want to be able to write many lines of code in a single cell, pressing the “return” key when in edit mode (green) moves the cursor to the next line in the cell just like in a text editor.
• We need some other way to tell the Notebook we want to run what’s in the cell.
• Pressing the “shift” and the “enter” key together will execute the contents of the cell.
• Notice that the “return” and “shift” keys on the right of the keyboard are right next to each other.

Use comments to add documentation to programs.

# This sentence isn't executed by Python.
adjustment = 0.5   # Neither is this - anything after '#' is ignored.

More Math

What is displayed when a Python cell in a notebook that contains several calculations is executed? For example, what happens when this cell is executed?

7 * 3
2 + 1

Solution

Python returns the output of the last calculation.

3

Key Points

• Python programs are plain text files.

• Use the Jupyter Notebook for editing and running Python.

• The Notebook has Command and Edit modes.

• Use the keyboard and mouse to select and edit cells.

• The Notebook will turn Markdown into pretty-printed documentation.

• Markdown does most of what HTML does.