Add Slot In Qt Designer
- Categories: python , tutorials
- #pyqt , #python-3 , #qt , #rad
- 9 minutes read
Confession: I am the opposite of the lazy coder. I like doing things thehard way. Whether it’s developing Java in Vim without code completion,running JUnit tests on the command line (don’t forget to specify all 42dependencies in the colon-separated classpath!), creating a LaTeX graphthat looks “just right”, or writing sqlplus scripts instead of usingSQL Developer (GUIs are for amateurs), I always assumed that doing sowould make me a better programmer.
So when I was just a fledgling programmer, and I had to design andcreate some dialog windows for a side project of mine (an awesomeadd-on toAnkiSRS written in Python and Qt), I didwhat I always do: find a good resource to learnPyQt and then code everything by hand.Now, since Python is a dynamic language and I used to develop thisadd-on in Vim, I had no code completion whatsoever, and only the Qtdocumentation and that tutorial to go by. Making things even moreinteresting, is that the documentation on Qt is in C++, and is full ofstuff like this:
Obviously, I had no idea what all the asterisks and ampersands meant andhad to use good-old trial and error to see what would stick in Python.Now, on the plus side, I experimented quite a lot with the code, but nothaving code completion makes it really hard to learn the API. (And evennow, using PyCharm, code completion will not always work, because ofPython being a dynamically-typed language and all. I am still looking atthe Qt documentation quite a bit.)
When we save our project in Qt Designer, it will create a.ui file, which is just XML containing all the properties (widgets, sizes, signals & slots, etc.) that make up the GUI. PyQt comes with a program, pyuic4, that can convert these.ui files to Python code. Maybe it'll help. By default you have to choose from the existing list of slots. But you can add slot by right-clicking at you object in the list at right side of designer and choose 'slot/signals' and add your custom slot/signal. After that, you can choose it in signal/slot editor.
One thing I noticed, though, is that the guy who develops Anki, DamienElmes, had all these .ui files lying around,and a bunch more files that read: “WARNING! All changes made to thisfile will be lost!”. Ugh, generated code! None of the dedicated,soul-cleansing and honest hard work that will shape you as a softwaredeveloper. It goes without saying I stayed far away from that kind oflaziness.
It took me some time to come around on this. One day, I actually got ajob as a professional software developer and I had much less time towork on my side-projects. So, when I wanted to implement another featurefor my Anki add-on, I found I had too little time to do it theold-fashioned way, and decided to give Qt Designer a go. Surprisingly, Ifound out it can actually help you tremendously to learn how Qt works(and also save you a bunch of time). Qt Designer allows you to visuallycreate windows using a drag-and-drop interface, then spews out an XMLrepresentation of that GUI, which can be converted to code. Thatgenerated code will show you possibilities that would have taken a longtime and a lot of StackOverflowing to figure out on your own.
So, to help other people discover the wonders of this program, here is alittle tutorial on how to do RAD and how to get a dialog window up andrunning with Qt Designer 4 and Python 3.
Installation
First, we need to install Qt Designer. On Arch Linux, it’s part of theqt4 package and you’ll also need python-pyqt4 for the Pythonbindings. (On Ubuntu, you have to install both qt4-designer and thepython-qt4 packages.)
Our goal
Our goal is to create a simple dialog window that has a text input fieldwhere we can type our name, and have a label that will display “Hellothere, $userName”. Yes, things will be that exciting. Along the way,we will learn how to assign emitted signals to slots and how to handleevents.
Creating a dialog
Fire up Qt Designer, and you will be presented with a “New form” dialog(if you do not see it, go to File > New…).
For this tutorial, we are going to choose a fairly small “Dialog withButtons Bottom”:
To the left are the widgets that we can add to our freshly createddialog, to the right, from top to bottom, we see the currently addedwidgets, the properties of those widgets and the signals and slotscurrently assigned to the dialog window.
I’m going to add a text label and a so-called line editor to our widget.To make sure they will align nicely, I will put them together in ahorizontal container, a QHBoxLayout:
In the object inspector, we can see the hierarchy of added widgets:
This is all simple drag-and-drop: we select a widget from the left pane,drag it to the desired location in the dialog and release the mouse.
Finally, we add two more label: one at the top, instructing the userwhat to do, and a second one near the bottom, where we soon will displayour message.
Qt Designer provides an easy way to connect signals to slots. If you goto Edit > Edit Signals/Slots (or press F4) you will be presented witha graphical overview of the currently assigned signals and slots. Whenwe start out, the button box at the bottom already emits two signals:rejected and accepted, from the Cancel and Ok button respectively:
Add Slot In Qt Designer Download
The signals rejected() and accepted() are emitted when the canceland OK buttons are clicked respectively, and the ground symbols indicatethe object that is interested in these signals: in this case the dialogwindow itself. Signals are handled by slots, and so the QDialog willneed to have slots for these signals. The slots (or handlers) are namedreject() and accept() in this instance, and since they are defaultslots provided by Qt, they already exist in QDialog, so we won’t haveto do anything (except if we want to override their default behavior).
What we want to do now is “catch” the textEdited signal from the lineeditor widget and create a slot in the dialog window that will handleit. This new slot we’ll call say_hello. So we click the line editorwidget and drag a line to anywhere on the dialog window: a ground symbolshould be visible:
In the window that appears now, we can select the signal that we areinterested in (textEdited) and assign it to a predefined slot, or wecan click on Edit… and create our own slot. Let’s do that:
We click the green plus sign, and type in the name of our new slot(say_hello):
The result will now look like:
In Qt Designer, you can preview your creation by going to Form >Preview… (or pressing Ctrl + R). Notice, however, that typing text inthe line editor won’t do anything yet. This is because we haven’twritten any implementation code for it. In the next section we will seehow to do that.
You can also see the code that will be generated by going to Form >View code…, although this code is going to be in C++.
Okay, enough with designing our dialog window, let’s get to actualPython coding.
Generating code (or not)
When we save our project in Qt Designer, it will create a .ui file,which is just XML containing all the properties (widgets, sizes, signals& slots, etc.) that make up the GUI.
PyQt comes with a program, pyuic4, that can convert these .ui filesto Python code. Two interesting command-line options are --preview (or-p for short), that will allow you to preview the dynamically createdGUI, and --execute (or -x for short), that will generate Python codethat can be executed as a stand-alone. The --output switch (or -o)allows you to specify a filename where the code will be saved.
In our case, creating a stand-alone executable is not going to work,because we have added a custom slot (say_hello) that needs to beimplemented first. So we will use pyuic4 to generate the form class:
This is the result:
It is here that you can learn a lot about how PyQt works, even thoughsome statements seem a bit baroque, like the binding of thetextChanged signal to our custom slot:
If you would write this yourself, you would probably prefer:
(Incidentally, the text between the square brackets intextChanged[str] indicates that a single argument of this type ispassed to the slot.)
Bringing it all together
Having generated our form class, we can now create a base class thatwill use this class:
When we are passing self to self.ui.setupUi, we are passing thewidget (a QDialog) in which the user interface will be created.
We implement our custom slot by defining a method say_hello that takesa single argument (the user-provided text from the line editor). Wecraft a witty sentence with it, and set it as the label text.
As mentioned before, we could also dynamically create the GUI directlyfrom the .ui file:
Running the application
Add Slot In Qt Designer Software
Running this will show the following (drum roll):