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Getting Started with Python and Raspberry Pi (Redirected from Learning Python By Developing Raspberry Pi Applications)

Chapter 1. Your First Steps with Python on the Pi

In this chapter, we will look at setting up the Raspbian operating system on the Raspberry Pi and have a quick look at the Python development tools that come pre-installed on it, along with looking at some basic ways in which we can execute the Python code.

The only things that are required here are:

A Raspberry Pi
A USB power source capable of delivering at least 1A
USB keyboard
USB mouse
TV with HDMI port
An SD card (or microSD card for the model B+ and Pi 2) of at least 4GB capacity
An SD card reader
A USB hub (if you wish to connect more USB devices that there are ports on the Raspberry Pi)
Optionally, a WiFi adapter if you want to connect the Pi to your network wirelessly (the list of supported USB WiFi adapters is available at elinux.org/RPi_USB_Wi-Fi_Adapters)

Installing and setting up Raspbian

The first thing we need to do is head to the Raspberry Pi downloads page at https://www.raspberrypi.org/downloads/ and download the latest version of Raspbian. This is a version of the Debian Linux distribution, specifically designed for the Raspberry Pi.

On the downloads page select the Download ZIP option under RASPBIAN.
Once the file has downloaded, extract the Zip archive using the default tool on your OS. You should now have a single file ending with the file extension .img.

Writing to the SD card

The next step is to write the just downloaded operating system image to the SD card so that it can be used with the Pi. The way this is done varies depending on the operating system you use on your main PC.

Windows

On Windows, we will use a tool called Win32 Disk Imager to write the OS image to the SD card. This tool can be downloaded from the SourceForge page at sourceforge.net/projects/win32diskimager.

Once downloaded and installed, insert your SD card and open Win32 Disk Imager. You should see a window similar to the following screenshot:
The important thing to check is that a drive letter appears in the Device drop down list. If this does not happen then Win32 Disk Imager has failed to recognize your SD card. In such a case, try it in a different SD card reader. If it still does not work then it could indicate that the card has failed.
Next, browse to select the .img file you had previously extracted from the downloaded Zip archive and click the Write button as shown in the following screenshot, after first making sure that the correct device is selected in the Device drop down list:
You will then see a confirmation dialog similar to the one shown in the next screenshot, asking you to confirm that the image and device are correct. Assuming they are, click on Yes.
Win32 Disk Imager will now write the image file to the SD card. This can take a few minutes. Once complete, you will see a confirmation dialog box as seen in the following screenshot:

You now have Raspbian loaded on the SD card and can now move on to the Boot Pi for the first time. This will be covered in the following sections.

Linux and Mac

On Linux and Mac, the dd command line utility can be used to write the operating system image to the SD card.

First, we need to determine the path to the storage device you want to write to. On Linux, the easiest way to do this is by using the udev management tool to monitor the udev logs. This is done by using the following command:
```
udevadm monitor --udev
```
Now insert the SD card and you should see a series of log messages printed to the console, similar to those shown in the following image. The last few should contain the paths to the partitions already on the drive (in my case, /dev/sdb1 and /dev/sdb2; from this we can deduce that the path to the SD card is /dev/sdb).
Next, we need to ensure that none of the existing partitions are mounted before we try to write to the SD card. This can be done by running the following command for every partition discovered using udevadm:
```
umount PATH
```
Here PATH is the path to the partition. This should give an output similar to the following image if the partition was not mounted; otherwise the command will exit without printing any output:
At this point, the SD card is ready to be written to. For this we will use the following command:
```
sudo dd if=[path to .img] of=[path to SD]
```
Here [path to .img] is the path to the .img file extracted from the Zip archive downloaded earlier and [path to SD] is the path to the SD card we just discovered.
This process will take some time (up to 20 minutes) and is complete when the command exits and you see the next shell prompt as shown in the following screenshot. If the writing fails then an error message will be printed to the terminal.

Booting the Pi for the first time

Now that you have an SD card with Raspbian installed on it, you are ready to boot the Pi for the first time and perform the first time configuration steps required to get the Pi up and running.

Note that to fully setup the Pi, you will need to have a way to connect it to the internet in order to install and update the software packages. This can either be wired (using an Ethernet cable) or wireless (using a USB WiFi adapter).

Firstly, connect the mouse, keyboard, monitor, and either the WiFi adapter or the Ethernet cable to the Pi. Insert the SD card and connect a USB power source. You should see the red PWR LED (Light Emitting Diode) light up and shortly after that, the green ACT LED would start to blink.
Note that the USB power source should be able to supply at least 1.5A to ensure reliable operation of the Pi. Usually, the USB chargers supplied with the tablets are a good choice of power supply.
Once the Pi has booted, you will see the configuration utility as shown in the following screenshot. The first thing we need to do here is to expand the root partition on the SD card to fill the entire SD card. This ensures that we have the maximum space available once we start using the Pi. This is done by selecting the Expand Filesystem option at the top of the list and pressing Enter.
Once the filesystem has been modified, you will see a message similar to the one shown next. Press Enter to return to the main menu.
Next, we will change the password for the default Pi user. This is done by selecting the second option on the main menu, Change User Password, and pressing Enter.
You will now see a message box similar to the one shown next, with instructions on entering a new password. Press Enter to continue.
You will now be required to enter a new password. Press Enter when finished. Once you have done this, you will be asked to enter the password again to confirm.
Note
Note that when entering a password you will not see any characters appear on the screen.
Now that the password has been changed, we need to set the default boot action to start LXDE, the desktop manager used on Raspbian. Select Enable Boot to Desktop/Scratch and press Enter.
Now select the second Desktop option and press Enter.
You may also wish to change the default locale using the Internationalisation Options menu option. By default, the Pi is configured for the UK.
Once you are ready to reboot the Pi to apply all of the new settings. This is done by selecting the Finish option and pressing Enter.
You will be asked for a confirmation that you want to reboot. Select Yes and press Enter.
If you are using Ethernet to connect to your network then you can skip this step. Otherwise, we will now setup the WiFi adapter and connect to a wireless network.
1. Open wpa_gui by choosing the WiFi Configuration utility in the Preferences submenu from the main menu in Raspbian.
2. Click on Scan to search for wireless networks in range. When complete, you should see a list similar to the one in the following screenshot:
3. When the scan completes, double click on the WiFi network you wish to connect to and you will be shown a window similar to the one in the following image, with some of the details of the network filled in:
4. Here all that is usually needed to be done is to enter the WiFi password in the PSK field and click on Add.
5. When done, the network should be selected in the Network drop down box automatically. Now click on Connect to connect to the network. Assuming all went well, you should see the Status of the connection show Connected, as shown in this next screenshot:
Now that we have an internet connection on the Pi, the final setup is to update the software packages already installed on the Pi. This can be done by opening a terminal, by clicking on the black monitor in the top left corner of the screen and typing the following commands:
```
sudo apt-get update
sudo apt-get upgrade
```
Each of these commands will take a few minutes to execute. The first updates the list of the available packages and the second updates each of the installed packages to the latest version.

Now that we have Raspbian setup on the Pi, we can move on to having a look at some of the tools we can use to write and execute Python scripts on the Pi.

The Python development tools

Now that the Pi is set up and running Raspbian, we can have a look at some of the tools we will use to develop Python scripts (small text files containing commands) and applications. Most of the time we will be using either the interactive Python terminal to execute the code line by line or the python executable to run full scripts and applications.

We will first look at the interactive terminal. First open a terminal by clicking on the black monitor icon in the top right corner of the desktop. This will open an LXTerminal window. In this window, type python and press Enter. This will start the interactive terminal as shown in the following screenshot:

From here we can type the Python code line by line; each line is executed as soon as it is typed, making this tool useful for quick testing and debugging (I also find that it makes a nice command line calculator). To demonstrate this, type in the following code and press Enter:

print "Hello, world!"

This will print the test Hello, world! on the line next to where you typed it in, as shown in the following screenshot:

The python executable can also be used to run the existing Python script files (which have the .py file extension), which we will look at later in the chapter.

One alternative to the interactive terminal is the IDLE Integrated Development Environment (IDE) which can be used both as an interactive terminal and a source file editor, and provides syntax highlighting for the Python files. It can be found by selecting Python 2 from the Programming submenu of the main menu on Raspbian, as shown in the following screenshot:

When first opened, it will be in the interactive terminal mode and can be used in the same way as the terminal ran from the command line, as shown in the following screenshot:

Python 2 versus Python 3

You will notice that there are two versions of Python installed by default on Raspbian: Python 2.7 and Python 3.1. Whilst the fundamentals of Python programming have not greatly changed between the two versions, there are notable differences that may prevent a code that was written for one version from working when executed with the interpreter for a different version.

For this reason, we will only use Python 2.7 in this book as this has the widest library support and is still the default Python version on many operating systems.

Note

More information of the differences between Python versions is available on the Python Wiki at wiki.python.org/moin/Python2orPython3

What you will learn

Fundamentals of Python applications Designing applications for multithreading Interacting with electronics and physical devices Debugging applications when they go wrong Packaging and installing Python modules User interface design using Qt Building easy to use commandline interfaces Connecting applications to the Internet

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