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Python for Beginners 2015.11.01

Getting started¶

What you’ll need¶

A Python!¶

If you haven’t yet got python, the latest official installation packages can be found here:

http://python.org/download/

Python 3 is preferable, being the newest version out!

Note

On Windows, you’ll want to add Python to your PATH, so it can be found by other programs. With Python 3.5 or later, there should be and option to do this in the installer. Otherwise, you can navigate to your installation directory (C:\Python34\), open the Tools, then Scripts folder, and run the win_add2path.py file by double clicking on it.

And a Code Editor¶

A code editor helps with reading and writing programming code. There are many around, and it is one of the most personal choices a programmer can make - Like a tennis-player choosing their racket, or a chef choosing their favourite knife. To start off with, you’ll just want a basic, easy-to-use one that doesn’t get in your way, but is still effective at writing python code. Here are some suggestions for those:

Atom: A new code editor available for Windows, Mac and Linux. It’s an open-source project developed by GitHub and is very easy to add functionality for, with its packages system.
Sublime Text: A great all around editor that’s easy to use. It’s Ctl+B shortcut lets you run the python file you’re working on straight away. Runs on Windows, Mac and Linux.
Geany: A simple editor that doesn’t aim to be too complicated. Available on Windows and Linux (you can probably find it in your package manager).
TextMate: One of the most famous code editors for Mac, it used to be a paid product but has since been open-sourced.
Gedit and Kate: if you run Linux using Gnome or KDE respectively, you might already have one of these two installed!
Komodo Edit: a sleak, free editor for Mac, Windows and Linux, based on the more powerful Komodo IDE.

If you’d like our recommendation, try out Sublime Text first.

Tip

Wordpad, TextEdit, Notepad, and Word are not suitable code editors.

What is Python, exactly?¶

Ok, so python is this thing called a programming language. It takes text that you’ve written (usually referred to as code), turns it into instructions for your computer, and runs those instructions. We’ll be learning how to write code to do cool and useful stuff. No longer will you be bound to use others’ programs to do things with your computer - you can make your own!

Practically, Python is just another program on your computer. The first thing to learn is how to use and interact with it. There are in fact many ways to do this; the first one to learn is to interact with python’s interpreter, using your operating system’s (OS) console.

A console (or ‘terminal’, or ‘command prompt’) is a textual way to interact with your OS, just as the ‘desktop’, in conjunction with your mouse, is the graphical way to interact your system.

Opening a console on Mac OS X¶

OS X’s standard console is a program called Terminal. Open Terminal by navigating to Applications, then Utilities, then double-click the Terminal program. You can also easily search for it in the system search tool in the top right.

The command line Terminal is a tool for interacting with your computer. A window will open with a command line prompt message, something like this:

mycomputer:~ myusername$

Opening a console on Linux¶

Different linux distributions (e.g Ubuntu, Fedora, Mint) may have different console programs, usually referred to as a terminal. The exact terminal you start up, and how, can depend on your distribution. On Ubuntu, you will likely want to open Gnome Terminal. It should present a prompt like this:

myusername@mycomputer:~$

Opening a console on Windows¶

Window’s console is called the Command Prompt, named cmd. An easy way to get to it is by using the key combination Windows+R (Windows meaning the windows logo button), which should open a Run dialog. Then type cmd and hit Enter or click Ok. You can also search for it from the start menu. It should look like:

C:\Users\myusername>

Window’s Command Prompt is not quite as powerful as its counterparts on Linux and OS X, so you might like to start the Python Interpreter (see just below) directly, or using the IDLE program that Python comes with. You can find these in the Start menu.

Using Python¶

The python program that you have installed will by default act as something called an interpreter. An interpreter takes text commands and runs them as you enter them - very handy for trying things out.

Just type python at your console, hit Enter, and you should enter Python’s Interpreter.

To find out which version of python you’re running, instead type python -V in your console to tell you.

Interacting With Python¶

After Python opens, it will show you some contextual information similar to this:

Python 3.5.0 (default, Sep 20 2015, 11:28:25)
[GCC 5.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Note

The prompt >>> on the last line indicates that you are now in an interactive Python interpeter session, also called the “Python shell”. This is different from the normal terminal command prompt!

You can now enter some code for python to run. Try:

print("Hello world")

Press Enter and see what happens. After showing the results, Python will bring you back to the interactive prompt, where you could enter another command:

>>> print("Hello world")
Hello world
>>> (1 + 4) * 2
10

An extremely useful command is help(), which enters a help functionality to explore all the stuff python lets you do, right from the interpreter. Press q to close the help window and return to the Python prompt.

To leave the interactive shell and go back to the console (the system shell), press Ctrl-Z and then Enter on Windows, or Ctrl-D on OS X or Linux. Alternatively, you could also run the python command exit()!

Exercise¶

Just above we demonstrated entering a command to figure out some math. Try some math commands of your own! What operations does python know? Get it to tell you what 239 and 588 added together, and then squared is.

Solution¶

Here are some ways you might have got the answer:

>>> 239 + 588
827
>>> 827 * 827
683929

>>> (239 + 588) * (239 + 588)
683929

>>> (239 + 588) ** 2
683929

Running Python files¶

When you have a lot of python code to run, you will want to save it into a file, so for instance, you can modify small parts of it (fix a bug) and re-run the code without having to repeatedly re-type the rest. Instead of typing commands in one-by-one you can save your code to a file and pass the file name to the python program. It will execute that file’s code instead of launching its interactive interpreter.

Let’s try that! Create a file hello.py in your current directory with your favorite code editor and write the print command from above. Now save that file. On Linux or OS X, you can also run touch hello.py to create an empty file to edit. To run this file with python, it’s pretty easy:

$ python hello.py

Note

Make sure you are at your system command prompt, which will have $ or > at the end, not at python’s (which has >>> instead)!

On Windows you should also be able to double-click the Python file to run it.

When pressing Enter now, the file is executed and you see the output as before. But this time, after Python finished executing all commands from that file it exits back to the system command prompt, instead of going back to the interactive shell.

And now we are all set and can get started with turtle!

Note

Not getting “Hello world” but some crazy error about “can’t open file” or “No such file or directory?” Your command line might not be running in the directory that you saved the file in. You can change the working directory of your current command line with the cd command, which stands for “change directory”. On Windows, you might want something like:

> cd Desktop\Python_Exercises

On Linux or OS X, you might want something like:

$ cd Desktop/Python_Exercises

This changes to the directory Python_Exercises under the Desktop folder (yours might be somewhere different). If you don’t know the location of the directory where you saved the file, you can simply drag the directory to the command line window. If you don’t know which directory your shell is currently running in use pwd, which stands for “print working directory”.

Warning

When playing around with turtle, avoid naming your file turtle.py — rather use more appropriate names such as square.py or rectangle.py. Otherwise, whenever you refer to turtle, Python will pick up your file instead of the standard Python turtle module.

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