Run it like you own it
The Script Runner allows you to execute a script in QGIS to automate a task. The script can use two approaches to access the QGIS API:
Use the qgis.utils.iface object Import qgis.core and qgis.gui
In the first method, you are limited to to only those methods supported by the QgisInterface class. In the second you have full access to the PyQGIS API.
In order for Script Runner to execute your script you must define a run_script function that accepts a single argument. This is the standard entry point used by Script Runner. A reference to the qgis.utils.iface object will be passed to your run_script function. You don't have to use the iface object in your script but your run_script function must accept it as an argument.
Here is an example of a simple run_script function:
def run_script(iface):
ldr = Loader(iface)
ldr.load_shapefiles('/vmap0_shapefiles')
In this example, the run_script creates an instance (ldr) of a class named Loader that is defined in the same source file. It then calls a method in the Loader class named load_shapefiles to do something useful---in this case, load all the shapefiles in a specified directory.
Alternatively, you could choose not to use classes and just do everything within the run_script function, including having it call functions in the same script or others you might import. The important thing is to be sure you have defined a run_script function. If not, Script Runner won't load your script.
To run a script, you must add it to Script Runner using the Add Script tool on the toolbar. This will add it to a list in the left panel. This list of scripts is persisted between uses of QGIS. You can remove a script using the Remove Script tool. This just removes it from the list; it does nothing to the script file on disk.
Once you have a script loaded, you can click the Script Info tool to populate the Info and Source tabs in the panel on the right. The Info tab contains the docstring from your module and then a list of the classes, methods, and functions found in the script. Having a proper docstring at the head of your script will help you determine the puprose of script.
You can view the source of the script on the Source tab. This allows you to quickly confirm that you are using the right script and it does what you think it will.