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Installation Guide and Running your First Python Script

Installing Python on a Mac

Linux computers come with a package manager like apt and yum that can download software for you, but OS X is missing one. That's where Homebrew comes in (https://brew.sh).

  • Open a new terminal and enter /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
  • Then, enter brew install python3
  • Done!

Python 2.7 comes bundled with Macs already, so be careful to use python3 and pip3 or you might encounter unexpected errors.

Installing Python in Windows or Linux

Running Python in Windows or Linux

  • You can use the command line to run codes, but for learning, the IDLE program (packaged with Python) can be helpful.
  • When you run IDLE, you'll automatically be in the interactive interpreter mode where you can run Python commands one at a time.
    • To create a new file from IDLE, go to File > New Window. A new, blank screen will open up where you can create your Python projects.
    • When you have a Python file open in IDLE, you can run it at any time by pressing F5.
  • From the command line, run python3 shell and you'll enter the interpreter where you can run Python commands one at a time, similar to IDLE.
    • You can create Python files in any text editor, and run them in the command line by writing python3 and then the path to your file.

You should probably make it easy on yourself and download Sublime Text 3, which will allow you to run Python files straight from your editor.

Mac: Running Python commands one at a time (using the interactive interpreter)

  • Open a terminal window (from the Finder, search for Terminal)
  • Type python3 and hit enter
  • Your command prompt should change from a dollar sign ($) to three greater than signs (>>>)
  • The command prompt is a signal of where you are -- terminal is $; python is >>>
  • Once you have the >>> prompt, you can enter as many Python commands as you like.
  • Type exit() to quit and return to the command prompt.

Mac: Running Python Files from the Terminal

  • Open a terminal window (from the Finder, search for Terminal)
  • When you open the terminal window, the words before your username are the folder you're in.
  • You're probably on your desktop to start. I recommend creating a folder on your desktop to put your Python scripts in.
  • For this example, let's say I named the folder pyscripts.
  • Go back to the terminal and type in cd pyscripts -- this will change the directory the terminal is working in to pyscripts.
  • When you create a Python script for Python to run, you should create it in TextEdit, TextWrangler, or some other text editor that can save files in plain text format -- any other kind of format will confuse Python!
  • Let's also assume that I created a Python script called lesson1_pbj.py, and that file lives in pyscripts on the desktop.
  • To run lesson1_pbj.py, I would type into the terminal: python3 lesson1_pbj.py

Mac: Troubleshooting Running Python Files from the Terminal

  • Make sure you're in the correct folder! You can check which folder the terminal is looking in by typing pwd
  • Similarly, make sure the file you want to run is in the folder you're in! Type ls to see all of the files in the folder terminal is currently working in.
  • Remember that when you see the dollar sign prompt ($), you're working in terminal, and can run scripts at the command line. But when you see the >>> prompt, you're in Python's interactive interpreter mode, and you'll need to type exit() to get back.