Big steps

1. To create your plugin... install plugins !
    
2. Create an user interface (ui)
    
3. Import your code and link it to your ui

1.Plugins

Plugin builder

Help your plugin creation.

Gives an usable template, and an empty UI.

This is your first step to your first plugin.

Reloader

You don't need anymore to reboot Qgis when you make a modification in your script.

How plugin works ?

main file.py

interface.py

resources.py

interface.ui

resources.qrc

pyuic4

pyrcc4

Very simplified schema

2.Interface creation

Open Qt Designer

Load empty UI generated with Plugin Builder and add classes :

Once your ui is done :

• Open ui file as text and remove resources.qrc tag if exists (and if you use images)
• Compile resources.qrc in python (using pyrcc4 )
       pyrcc4 resources.qrc -o resources.py
• Compile your ui in python (using command pyuic4)
       pyuic4 my_first_plugin_dialog_base.ui -o  my_first_plugin_dialog_base.py

2.Interface creation

Download PyQt4 (binary packages):

https://www.riverbankcomputing.com/software/pyqt/download

If you used plugin builder, change call for your ui (cf my post)

3. Link code/ui

Open your main file.py (ie my_first_plugin.py)

In import PyQt4.QtGui add QDialog and QFileDialog :

QFileDialog is loading/saving window

QDialog is to know for where the action comes from

Add QDialog in your main class dependency, then init QDialog:

from PyQt4.QtGui import QDialog, QFileDialog

myfirstplugin( QDialog ) :
    QDialog.__init__(self)     

3. Link code/ui

Import QgsMessageLog for a perfect debugging

Then you create a log message with :

from qgis.core import QgsMessageLog

QgsMessageLog.logMessage('bug using : '+yourVariabletoTest)

Once QDialog is init, you can use self.sender()

It gives you the origin of an action because you can't use args

​Example : you clic on the "runButton" and self.dlg is your ui() :

3. Link code/ui

self.dlg.runButton.clicked.connect(self.saveTo)

def saveTo(self):
    if self.sender() == self.dlg.runButton:
        #do the job

         fileName = QFileDialog.getSaveFileName(self.dlg, \
         "Select output file","","TIF (*.tif)")

Then you can open a window that let the user select the file to save :

Now, what to do with your filename ?

Save it in a QLineEdit field.

If your file outRaster is a QLineEdit field, you can show to the user his file choice :

3. Link code/ui

self.dlg.outRaster.setText(fileName)

outRasterVariable = self.dlg.outRaster.text()

Then you can assign it to a variable 

Well done !

You have now all basics to make your own Qgis plugin.

Some tips :

try:
    from osgeo import gdal
except:
    QgsMessageLog.logMessage('Gdal is not available')

Remember to use QgsMessageLog to debbug

Make sure your script runs in python first, then merge it in your plugin

Use self.sender() from QDialog to replace args from ui

To avoid bugs, use try/except function