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PSA: Please use new style Qt signals and slots not the old style

April 24, 2015 Nathans QGIS and GIS blog

Don’t do this:

self.connect(self.widget, 
             SIGNAL("valueChanged(int)"), 
             self.valuechanged)

It’s the old way, the crappy way. It’s prone to error and typing mistakes. And who really wants to be typing strings as functions and arg names in it. Gross.

Do this:

self.widget.valueChanged.connect(self.valuechanged)
self.widget.valueChanged[str].connect(self.valuechanged)

Much nicer. Cleaner. Looks and feels like Python not some mash up between C++ and Python. The int argument is the default so it will use that. If you to pick the signal type you can use [type].

Don’t do this:

self.emit(SIGNAL("changed()", value1, value2))

Do this

class MyType(QObject):
   changed = pyqtSignal(str, int)

   def stuff(self):
       self.changed.emit(value1, value2)

pyqtSignal is a type you can use to define you signal. It will come with type checking, if you don’t want type checking just do pyqtSignal(object).

Please think of the poor kittens before using the old style in your code.


Filed under: pyqt, python, qgis Tagged: pyqt, qgis, qt

by Nathan at 6:23 AM under pyqt , python , qgis , qt (Comments)

Setting up PyCharm for PyQGIS and Qt

May 9, 2014 Nathans QGIS and GIS blog

I have been asked a few times how to setup PyCharm so you are able to do PyQGIS development, or even PyQt because that is just as great. Rather then tell each person one at a time I thought I would throw it out as a blog post so everyone gets the benift.

The first thing we need to do is create a batch file that will load PyCharm and setup all the paths correctly. We have to do this on Windows as Qt and QGIS are not on PATH. QGIS also ships with it's own version of copy of Python so we need to tell PyCharm about it.

The batch file is as simple as this:

SET OSGEO4W_ROOT=C:\OSGeo4W
SET QGISNAME=qgis
SET QGIS=%OSGEO4W_ROOT%\apps\%QGISNAME%
SET QGIS_PREFIX_PATH=%QGIS%
SET PYCHARM="C:\Program Files (x86)\JetBrains\PyCharm 3.0\bin\pycharm.exe"

CALL %OSGEO4W_ROOT%\bin\o4w_env.bat

SET PATH=%PATH%;%QGIS%\bin
SET PYTHONPATH=%QGIS%\python;%PYTHONPATH%

start "PyCharm aware of QGIS" /B %PYCHARM% %*

Save this somewhere called pycharm-pyqgis.bat and run it.

This is a pretty basic batch file. It just sets the variables that we need for the QGIS and Qt libs, and also sets the PYTHONHOME to the QGIS version. The magic sause here is the set PATH, set PYTHONHOM, and set PYTHONPATH variables. You can just update the OSGeo4W_ROOT, and PYCHARM variables for your setup.

After running the batch we need to setup a Python interpreter in PyCharm. Click Configure -> Settings on the load page (or settings in the File menu). Search for Python Interpreters, and press the green add button and select local. Here we need to add the Python interpreter that we setup in our batch file. In the one I posted above it will be found at C:\OSGeo4W\bin\python.exe. Press Ok and PyCharm will find all the python paths it needs for the setup.

Alt Text

Leave the settings dialog and create a new project.

Alt Text

Remember to select the interpreter that we setup for the installed version of QGIS.

That is pretty much it. We can now create a pyqgis and pyqt app in PyCharm.

Lets give it a go

from qgis.core import QgsApplication
from PyQt4.QtGui import QDialog

GUIEnabled=True
app = QgsApplication([], GUIEnabled)

dlg = QDialog()
dlg.exec_()

app.exit(app.exec_())

Run it in PyCharm with Alt+Shift+F10. Good to go!

by Nathan Woodrow at 3:00 PM under [dev , pycharm , pyqgis] , qt

Setting up PyCharm for PyQGIS and Qt

I have been asked a few times how to setup PyCharm so you are able to do PyQGIS development, or even PyQt because that is just as great. Rather then tell each person one at a time I thought I would throw it out as a blog post so everyone gets the benift.

The first thing we need to do is create a batch file that will load PyCharm and setup all the paths correctly. We have to do this on Windows as Qt and QGIS are not on PATH. QGIS also ships with it's own version of copy of Python so we need to tell PyCharm about it.

The batch file is as simple as this:

SET OSGEO4W_ROOT=C:\OSGeo4W
SET QGISNAME=qgis
SET QGIS=%OSGEO4W_ROOT%\apps\%QGISNAME%
SET QGIS_PREFIX_PATH=%QGIS%
SET PYCHARM="C:\Program Files (x86)\JetBrains\PyCharm 3.0\bin\pycharm.exe"

CALL %OSGEO4W_ROOT%\bin\o4w_env.bat

SET PATH=%PATH%;%QGIS%\bin
SET PYTHONPATH=%QGIS%\python;%PYTHONPATH%

start "PyCharm aware of QGIS" /B %PYCHARM% %*

Save this somewhere called pycharm-pyqgis.bat and run it.

This is a pretty basic batch file. It just sets the variables that we need for the QGIS and Qt libs, and also sets the PYTHONHOME to the QGIS version. The magic sause here is the set PATH, set PYTHONHOM, and set PYTHONPATH variables. You can just update the OSGeo4W_ROOT, and PYCHARM variables for your setup.

After running the batch we need to setup a Python interpreter in PyCharm. Click Configure -> Settings on the load page (or settings in the File menu). Search for Python Interpreters, and press the green add button and select local. Here we need to add the Python interpreter that we setup in our batch file. In the one I posted above it will be found at C:\OSGeo4W\bin\python.exe. Press Ok and PyCharm will find all the python paths it needs for the setup.

Alt Text

Leave the settings dialog and create a new project.

Alt Text

Remember to select the interpreter that we setup for the installed version of QGIS.

That is pretty much it. We can now create a pyqgis and pyqt app in PyCharm.

Lets give it a go

from qgis.core import QgsApplication
from PyQt4.QtGui import QDialog

GUIEnabled=True
app = QgsApplication([], GUIEnabled)

dlg = QDialog()
dlg.exec_()

app.exit(app.exec_())

Run it in PyCharm with Alt+Shift+F10. Good to go!

by Nathan Woodrow at 3:00 PM under [dev , pycharm , pyqgis] , qt
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