19.2. Lesson: Python Basics

In this lesson we will introduce you to the basics of python. If you have programmed with other languages (Java, C++, VB etc.) you will find that python is very easy and quick to learn, though it is a little different to the way other languages work, particularly in terms of its requirements for code formatting.

19.2.1. basic Follow Along: Hello World

Install python from python.org then open a terminal or command window and start the python prompt:

[email protected]:~/dev/cpp/QGIS-Training-Manual/python$ python

When it starts you will see a message like this:

Python 2.7.3 (default, Aug  1 2012, 05:14:39)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.

Now type print 'Hello World' at the command prompt as shown below:

>>> print 'Hello World'

Python will respond by running your command:

Hello World
>>>

Congratulations, you just wrote your first python application!

Note

You can escape from the python prompt by pressing ctrl-D or by typing quit() and then pressing Enter.

19.2.2. moderate Follow Along: Running commands from a file

Naturally it would be of limited use to only ever be able to type your python commands interactively, so it is common practice to save your python commands in a .py file and then run the file. For example, save this line in a text file called hello_world.py:

print 'Hello World'

Note

By convention, avoid saving your python code with file names containing spaces or hypens.

Now you can run your program by typing this from your command prompt:

python hello_world.py

19.2.3. hard Follow Along: Defining the interpreter in the file

It would be more convenient if we could just run the file directly. You can do this on Linux and Mac OSX by adding an interpreter annotation to the top of the file:

#!/usr/bin/python

print 'Hello World'

You will also need to make the file executable like this:

chmod +x hello_world.py

Now you can execute the file like this:

./hello_world.py

Note

Doing this may prevent your program from being portable accross operating systems.