Qt Designer for Python


Code for setting up individual widgets and the layout of a view can become large and difficult to maintain by hand. It usually easier to edit such code using a drag and drop WYSIWYG (What You See Is What You Get) editor such as Qt Creator/Designer. However, doing so requires some additional actions at build time.


Qt Creator was not originally designed to work with Python, and it is therefore not possible to directly save or export the layout as a Python script. Instead you must first save the layout in a .ui file and use the pyuic4 tool to convert it to a python script.

Integration With CMake

Running this tool manually for each ui file in the project would quickly become infeasible. Fortunately it is easy to integrate this with the cmake build using the UiToPy function defined in buildconfig/CMake/UiToPy.cmake. This function takes a list of ui files and a name to be used for a cmake target. It will produce a target with the specified name, which, when built runs the pyuic4 command on each of the .ui files to generate a .py file with a ui_ prefix in the same directory.

For example the following CMakeLists.txt:


UiToPy(UI_FILES CompileUISANSDataProcessorInterface)

Produces a cmake target CompileUISANSDataProcessorInterface which when built runs:

pyuic4 sans_data_processor_window.ui -o ui_sans_data_processor_window.py
pyuic4 settings_diagnostic_tab.ui -o ui_settings_diagnostic_tab.py
pyuic4 masking_table.ui -o ui_masking_table.py

The generated target also runs a script wrap_pyui.py which prepends #pylint: skip-file to the top of the generated file.

It is worth noting that for the main Mantid repository, in most cases include(UiToPy) can be omitted since the majority of Python GUIs have their .ui files under the scripts/Interface/ui directory and so scripts/Interface/ui/CMakeLists.txt performs this include.

Using the Generated Script

When following the MVP design pattern as described at MVP (Model View Presenter), the generated file alone is not sufficient as a View. Directly accessing the widgets and the signals defined on the view from the presenter moves the view implementation details into the presenter, which makes it harder to change the names and types of widgets used to display the information. Instead it is best to create a separate Python file which imports the generated one and adds a separate view class which inherits from the generated one.

import ui_add_runs_page # This imports the file generated by pyuic.

class AddRunsPage(QtGui.QWidget, ui_add_runs_page.Ui_AddRunsPage):

You can then add separate methods to the view for accessing and mutating the content of the widgets as well as add any necessary signals which form the interface to the view.

import ui_add_runs_page # This imports the file generated by pyuic.

class AddRunsPage(QtGui.QWidget, ui_add_runs_page.Ui_AddRunsPage):
    outFileChanged = pyqtSignal()

    def __init__(self, parent=None):
        super(AddRunsPage, self).__init__(parent)

    def _connect_signals(self):

    def out_file_name(self):
        return self.fileNameEdit.text().encode('utf-8')

Keeping GUIs modular using Widgets


When designing a GUI in QtCreator it is often too easy to end up with the entire interface in a single UI file. This can then lead to having a single presenter for the entire GUI and sometimes even a single model. This makes the UI harder to maintain as a whole and difficult to re-use, move and separate out individual sections.

Instead when building a GUI it is sometimes better to separate parts of the interface into smaller views and presenters which form a hierarchy of widgets. For example the new SANS Run Summation page is in it’s own UI file and uses two separate widgets - a RunSelectorWidget and a SummationSettingsWidget. Although these widgets are not currently used in any other interface, they are still isolated from the Run Summation tab and could easily be used in another interface should the need arise. The code is also better organised and more modular as a result of this clean separation.


Assuming we start with QtCreator with .ui file open which contains a section of an interface which we wish to move to its own widget. We must start by creating a new .ui file

1. Go to File > New File Or Project and select Qt Designer Form from the list of templates.

  1. Then select Widget from the list of form templates.

3. Enter the name for the file and save it to the location containing the corresponding CMakeLists.txt


4. Click Next and adjust any project management settings as you wish before clicking Finish.

At this point you should have an empty Widget in the design area.


You can switch between the two .ui files using the menu in the top left.

5. Next, copy the components you wish to move into the new widget and paste them into the new file.

_images/CopyFromMainUI.png _images/PasteIntoWidget.png

6. Make adjustments to the layout and resize behaviour of the widget as you see fit.


7. Add the following CMake snippet to your CMakeLists.txt, note that you may already have a target for generating the Python files in which case you can simply add your new .ui file to the list of existing .ui files.


UiToPy(UI_FILES CompileUIMyWidget)

Test that this has worked by saving your .ui file and re-running the build, the output should contain a line similar to the following:

[1/1] Generating scripts/Interface/ui/sans/ui_my_widget.py

8. Add a separate python file containing the View class which extends the generated one.

# my_widget.py
import ui_my_widget

class MyWidget(QtGui.QWidget, ui_add_runs_page.Ui_MyWidget):

9. Return to the original interface file, delete the components you copied across and replace them with a single Widget component found in the containers section.

_images/PreReplacedWidget.png _images/PostReplacedWidget.png

10. Right click on the newly created widget container and select Promote To…


11. For the Promoted Class Name field enter the name of the view class. If you are taking the advice given above, this should be the name of the class which inherits from the generated ui_my_widget.Ui_MyWidget class.

12. For the Header File field enter the fully qualified path of the python module which contains the class mentioned above.

13. You can leave the Global Include box un-ticked. Finish the promotion by pressing Add and then Promote.


14. Save your ui files, re-run and launch the build to see the finished result.