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While working on ape I had a problem with figuring out how to properly connect a signal to a slot, where the signal is emitted by a QTreeView widget. As this is not my first app with python and pyqt, I was doing something like (this is, btw, the “old style”):

• Pyqt New Style Signals And Slots Games
• Pyqt New Style Signals And Slots Free

but it simply didn’t work. Nothing happened. I was trying all different of connect/signal/slot combinations but everything was just dead silent. Google gave only pretty much old posts talking about QT3. Then I figured that, because the QTreeView is “sitting” inside a QDockWidget, maybe that dock widget thingy is somehow intercepting/taking over the signals. Nope. Wth? Wtf is going on? Current pyqt version is (on my machine) 4.6. Last time I used pyqt it was something like 4.2 or 4.3. Something must’ve been changed in the mean time. Off to the pyqt docs I go (btw, I use the official QT docs, the C++ version, there isn’t really a big difference from pyqt): PyQt reference, chapter 7 - 'New-style Signal and Slot Support'. A-ha! They changed it! Here is an example of the “new style”:

Slots and Signals. Defining custom slots and signals uses slightly different syntax between the two libraries. PySide2 provides this interface under the names Signal and Slot while PyQt5 provides these as pyqtSignal and pyqtSlot respectively. The behaviour of them both is identical for defining and slots and signals. Sending Python values with signals and slots. On the #pyqt channel on Freenode, Khertan asked about sending Python values via Qt's signals and slots mechanism. The following example uses the PyQtPyObject value declaration with an old-style signal-slot connection, and again when the signal is emitted, to communicate a Python dictionary.

Oh my, isn’t that just beautiful?! Much more readable and simpler, for me at least. And it works! Yay! The QTreeView signals are happily connected to slots, thus, I’m happy too.

PyQt is more versatile than C/Qt in this regard, because we can connect not just to slots, but also to any callable, and from PyQt 4.2, it is possible to dynamically add 'predefined' signals and slots to QObjects. Let's see how signals and slots works in practice with the Signals and Slots program shown in Figure 4.6. Connecting signals and slots with PyQt - the new style Robert Basic, software developer making web applications better Connecting signals and slots with PyQt - the new style published on November 09, 2010.

A few paragraphs later, turns out that the “old style” isn’t thrown out, it should still work. Why it didn’t work for me escapes me at the moment, but honestly, I don’t really care as long as the new style is working.

Happy hackin’!

• PyQt5 Tutorial
• PyQt5 Useful Resources

• Selected Reading

PyQt is a GUI widgets toolkit. It is a Python interface for Qt, one of the most powerful, and popular cross-platform GUI library. PyQt was developed by RiverBank Computing Ltd. The latest version of PyQt can be downloaded from its official website − riverbankcomputing.com

PyQt API is a set of modules containing a large number of classes and functions. While QtCore module contains non-GUI functionality for working with file and directory etc., QtGui module contains all the graphical controls. In addition, there are modules for working with XML (QtXml), SVG (QtSvg), and SQL (QtSql), etc.

A list of frequently used modules is given below −

• QtCore − Core non-GUI classes used by other modules

• QtGui − Graphical user interface components

• QtMultimedia − Classes for low-level multimedia programming

• QtNetwork − Classes for network programming

• QtOpenGL − OpenGL support classes

• QtScript − Classes for evaluating Qt Scripts

• QtSql − Classes for database integration using SQL

• QtSvg − Classes for displaying the contents of SVG files

• QtWebKit − Classes for rendering and editing HTML

• QtXml − Classes for handling XML

• QtWidgets − Classes for creating classic desktop-style UIs

• QtDesigner − Classes for extending Qt Designer

Supporting Environments

PyQt is compatible with all the popular operating systems including Windows, Linux, and Mac OS. It is dual licensed, available under GPL as well as commercial license. The latest stable version is PyQt5-5.13.2.

Windows

Wheels for 32-bit or 64-bit architecture are provided that are compatible with Python version 3.5 or later. The recommended way to install is using PIP utility −

To install development tools such as Qt Designer to support PyQt5 wheels, following is the command −

You can also build PyQt5 on Linux/macOS from the source code www.riverbankcomputing.com/static/Downloads/PyQt5

PyQt5 API is not automatically compatible with earlier versions. Hence, Python code involving PyQt4 modules should be upgraded manually by making relevant changes. In this chapter, main differences between PyQt4 and PyQt5 have been listed.

PyQt5 is not supported on versions of Python earlier than v2.6.

PyQt5 doesn't support connect() method of QObject class for connection between signal and slot. Hence the usage can no more be implemented −

Only the following syntax is defined −

Classes defined in earlier QtGui module have been distributed in QtGui, QtPrintSupport an QtWidgets modules.

In the new QFileDialog class, The getOpenFileNameAndFilter() method is replaced by getOpenFileName(), getOpenFileNamesAndFilter() by getOpenFileNames() and getSaveFileNameAndFilter() by getSaveFileName(). Older signatures of these methods also have changed.

PyQt5 doesn’t have provision to define a class that is sub-classed from more than one Qt class.

pyuic5 utility (to generates Python code from Designer's XML file) does not support the --pyqt3-wrapper flag.

pyrcc5 does not support the -py2 and -py3 flags. The output of pyrcc5 is compatible with all versions of Python v2.6 onwards.

PyQt5 always invokes sip.setdestroyonexit() automatically and calls the C++ destructor of all wrapped instances that it owns.

Pyqt New Style Signals And Slots Games

Creating a simple GUI application using PyQt involves the following steps −

• Import QtCore, QtGui and QtWidgets modules from PyQt5 package.

• Create an application object of QApplication class.

• A QWidget object creates top level window. Add QLabel object in it.

• Set the caption of label as 'hello world'.

• Define the size and position of window by setGeometry() method.

• Enter the mainloop of application by app.exec_() method.

Following is the code to execute Hello World program in PyQt −

The above code produces the following output −

It is also possible to develop an object oriented solution of the above code.

• Import QtCore, QtGui and QtWidgets modules from PyQt5 package.

• Create an application object of QApplication class.

• Declare window class based on QWidget class

• Add a QLabel object and set the caption of label as 'hello world'.

• Define the size and position of window by setGeometry() method.

• Enter the mainloop of application by app.exec_() method.

Following is the complete code of the object oriented solution −

PyQt API is a large collection of classes and methods. These classes are defined in more than 20 modules.

Following are some of the frequently used modules −

PyQt5 development tools is a collection of useful utilities for Qt development. Following is a select list of such utilities −

PyQt API contains more than 400 classes. The QObject class is at the top of class hierarchy. It is the base class of all Qt objects. Additionally, QPaintDevice class is the base class for all objects that can be painted.

QApplication class manages the main settings and control flow of a GUI application. It contains main event loop inside which events generated by window elements and other sources are processed and dispatched. It also handles system-wide and application-wide settings.

QWidget class, derived from QObject and QPaintDevice classes is the base class for all user interface objects. QDialog and QFrame classes are also derived from QWidget class. They have their own sub-class system.

Here is a select list of frequently used widgets

A typical GUI based application’s top level window is created by QMainWindow widget object. Some widgets as listed above take their appointed place in this main window, while others are placed in the central widget area using various layout managers.

The following diagram shows the QMainWindow framework −

The PyQt installer comes with a GUI builder tool called Qt Designer. Using its simple drag and drop interface, a GUI interface can be quickly built without having to write the code. It is however, not an IDE such as Visual Studio. Hence, Qt Designer does not have the facility to debug and build the application.

Start Qt Designer application which is a part of development tools and installed in scripts folder of the virtual environment.

Start designing GUI interface by choosing File → New menu.

You can then drag and drop required widgets from the widget box on the left pane. You can also assign value to properties of widget laid on the form.

The designed form is saved as demo.ui. This ui file contains XML representation of widgets and their properties in the design. This design is translated into Python equivalent by using pyuic5 command line utility. This utility is a wrapper for uic module of Qt toolkit. The usage of pyuic5 is as follows −

In the above command, -x switch adds a small amount of additional code to the generated Python script (from XML) so that it becomes a self-executable standalone application.

The resultant python script is executed to show the following dialog box −

The user can input data in input fields but clicking on Add button will not generate any action as it is not associated with any function. Reacting to user-generated response is called as event handling.

Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.

Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.

Using Qt Designer's Signal/Slot Editor

First design a simple form with a LineEdit control and a PushButton.

It is desired that if button is pressed, contents of text box should be erased. The QLineEdit widget has a clear() method for this purpose. Hence, the button’s clicked signal is to be connected to clear() method of the text box.

To start with, choose Edit signals/slots from Edit menu (or press F4). Then highlight the button with mouse and drag the cursor towards the textbox

As the mouse is released, a dialog showing signals of button and methods of slot will be displayed. Select clicked signal and clear() method

The Signal/Slot Editor window at bottom right will show the result −

Save ui and Build and Python code from ui file as shown in the below code −

Generated Python code will have the connection between signal and slot by the following statement −

Run signalslot.py and enter some text in the LineEdit. The text will be cleared if the button is pressed.

Building Signal-slot Connection

Instead of using Designer, you can directly establish signal-slot connection by following syntax −

Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following technique −

Example

In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.

When b1 is clicked, the clicked() signal is connected to b1_clicked() function −

When b2 is clicked, the clicked() signal is connected to b2_clicked() function.

The above code produces the following output −

Output

A GUI widget can be placed inside the container window by specifying its absolute coordinates measured in pixels. The coordinates are relative to the dimensions of the window defined by setGeometry() method.

setGeometry() syntax

In the following code snippet, the top level window of 300 by 200 pixels dimensions is displayed at position (10, 10) on the monitor.

A PushButton widget is added in the window and placed at a position 50 pixels towards right and 20 pixels below the top left position of the window.

This Absolute Positioning, however, is not suitable because of following reasons −

• The position of the widget does not change even if the window is resized.

• The appearance may not be uniform on different display devices with different resolutions.

• Modification in the layout is difficult as it may need redesigning the entire form.

PyQt API provides layout classes for more elegant management of positioning of widgets inside the container. The advantages of Layout managers over absolute positioning are −

• Widgets inside the window are automatically resized.

• Ensures uniform appearance on display devices with different resolutions.

• Adding or removing widget dynamically is possible without having to redesign.

Qt toolkit defines various layouts that can be used with Qt Designer utility.

Here is the list of Classes which we will discuss one by one in this chapter.

Here is the list of Widgets which we will discuss one by one in this chapter.

A QDialog widget presents a top level window mostly used to collect response from the user. It can be configured to be Modal (where it blocks its parent window) or Modeless (the dialog window can be bypassed).

PyQt API has a number of preconfigured Dialog widgets such as InputDialog, FileDialog, FontDialog, etc.

Example

In the following example, WindowModality attribute of Dialog window decides whether it is modal or modeless. Any one button on the dialog can be set to be default. The dialog is discarded by QDialog.reject() method when the user presses the Escape key.

A PushButton on a top level QWidget window, when clicked, produces a Dialog window. A Dialog box doesn’t have minimize and maximize controls on its title bar.

The user cannot relegate this dialog box in the background because its WindowModality is set to ApplicationModal.

The above code produces the following output. Click on button in main window and dialog box pops up −

QMessageBox is a commonly used modal dialog to display some informational message and optionally ask the user to respond by clicking any one of the standard buttons on it. Each standard button has a predefined caption, a role and returns a predefined hexadecimal number.

Important methods and enumerations associated with QMessageBox class are given in the following table −

Example

In the following example, click signal of the button on the top level window, the connected function displays the messagebox dialog.

setStandardButton() function displays desired buttons.

buttonClicked() signal is connected to a slot function, which identifies the caption of source of the signal.

The complete code for the example is as follows −

The above code produces the following output. Message Box pops up when main windows’ button is clicked −

If you click on Ok or Cancel button on MessageBox, the following output is produced on the console −

A typical GUI application may have multiple windows. Tabbed and stacked widgets allow to activate one such window at a time. However, many a times this approach may not be useful as view of other windows is hidden.

One way to display multiple windows simultaneously is to create them as independent windows. This is called as SDI (single Document Interface). This requires more memory resources as each window may have its own menu system, toolbar, etc.

MDI (Multiple Document Interface) applications consume lesser memory resources. The sub windows are laid down inside main container with relation to each other. The container widget is called QMdiArea.

QMdiArea widget generally occupies the central widget of QMainWondow object. Child windows in this area are instances of QMdiSubWindow class. It is possible to set any QWidget as the internal widget of subWindow object. Sub-windows in the MDI area can be arranged in cascaded or tile fashion.

The following table lists important methods of QMdiArea class and QMdiSubWindow class −

QMdiArea object emits subWindowActivated() signal whereas windowStateChanged() signal is emitted by QMdisubWindow object.

Example

In the following example, top level window comprising of QMainWindow has a menu and MdiArea.

Triggered() signal of the menu is connected to windowaction() function.

The new action of menu adds a subwindow in MDI area with a title having an incremental number to it.

Cascaded and tiled buttons of the menu arrange currently displayed subwindows in cascaded and tiled fashion respectively.

The complete code is as follows −

Pyqt New Style Signals And Slots Free

Run above code and three windows in cascased and tiled formation −

The provision of drag and drop is very intuitive for the user. It is found in many desktop applications where the user can copy or move objects from one window to another.

MIME based drag and drop data transfer is based on QDrag class. QMimeData objects associate the data with their corresponding MIME type. It is stored on clipboard and then used in the drag and drop process.

The following QMimeData class functions allow the MIME type to be detected and used conveniently.

Many QWidget objects support the drag and drop activity. Those that allow their data to be dragged have setDragEnabled() which must be set to true. On the other hand, the widgets should respond to the drag and drop events in order to store the data dragged into them.

• DragEnterEvent provides an event which is sent to the target widget as dragging action enters it.

• DragMoveEvent is used when the drag and drop action is in progress.

• DragLeaveEvent is generated as the drag and drop action leaves the widget.

• DropEvent, on the other hand, occurs when the drop is completed. The event’s proposed action can be accepted or rejected conditionally.

Example

In the following code, the DragEnterEvent verifies whether the MIME data of the event contains text. If yes, the event’s proposed action is accepted and the text is added as a new item in the ComboBox.

The above code produces the following output −

PyQt5 library contains QtSql module. It is an elaborate class system to communicate with many SQL based databases. Its QSqlDatabase provides access through a Connection object. Following is the list of currently available SQL drivers −

Example

For this chapter, a connection with a SQLite database is established using the static method −

Other methods of QSqlDatabase class are as follows −

QSqlQuery class has the functionality to execute and manipulate SQL commands. Both DDL and DML type of SQL queries can be executed. First step is to create SQlite database using the following statements −

Next, obtain Query object with QSqlQuery() method and call its most important method exec_(), which takes as an argument a string containing SQL statement to be executed.

The following script creates a SQLite database sports.db with a table of sportsperson populated with five records.

To confirm that the SQLite database is created with above records added in sportsmen table in it, use a SQLite Gui utility called SQLiteStudio.

QSqlTableModel class in PyQt is a high-level interface that provides editable data model for reading and writing records in a single table. This model is used to populate a QTableView object. It presents to the user a scrollable and editable view that can be put on any top level window.

A QSqlTableModel object is declared in the following manner −

Its editing strategy can be set to any of the following −

Example

In the following example, sportsperson table is used as a model and the strategy is set as −

QTableView class is part of Model/View framework in PyQt. The QTableView object is created as follows −

This QTableView object and two QPushButton widgets are added to the top level QDialog window. Clicked() signal of add button is connected to addrow() which performs insertRow() on the model table.

The Slot associated with the delete button executes a lambda function that deletes a row, which is selected by the user.

The complete code is as follows −

The above code produces the following output −

Try adding and deleting a few records and go back to SQLiteStudio to confirm the transactions.

All the QWidget classes in PyQt are sub classed from QPaintDevice class. A QPaintDevice is an abstraction of two dimensional space that can be drawn upon using a QPainter. Dimensions of paint device are measured in pixels starting from the top-left corner.

QPainter class performs low level painting on widgets and other paintable devices such as printer. Normally, it is used in widget’s paint event. The QPaintEvent occurs whenever the widget’s appearance is updated.

The painter is activated by calling the begin() method, while the end() method deactivates it. In between, the desired pattern is painted by suitable methods as listed in the following table.

Example

In the following code, various methods of PyQt's drawing methods are used.

The above code produces the following output −

In this chapter, we shall learn Brush Style Constants.

Brush Style Constants

Given below are the Brush Style Constants −

Predefined QColor Styles

Given below are the Predefined QColor Styles −

Predefined QColor Objects

Given below are the Predefined QColor Objects −

The QClipboard class provides access to system-wide clipboard that offers a simple mechanism to copy and paste data between applications. Its action is similar to QDrag class and uses similar data types.

QApplication class has a static method clipboard() which returns reference to clipboard object. Any type of MimeData can be copied to or pasted from the clipboard.

Following are the clipboard class methods that are commonly used −

Signal associated with clipboard object is −

Example

In the following example, two TextEdit objects and two Pushbuttons are added to a top level window.

To begin with the clipboard object is instantiated. Copy() method of textedit object copies the data onto the system clipboard. When the Paste button is clicked, it fetches the clipboard data and pastes it in other textedit object.

The above code produces the following output −

QPixmap class provides an off-screen representation of an image. It can be used as a QPaintDevice object or can be loaded into another widget, typically a label or button.

Qt API has another similar class QImage, which is optimized for I/O and other pixel manipulations. Pixmap, on the other hand, is optimized for showing it on screen. Both formats are interconvertible.

The types of image files that can be read into a QPixmap object are as follows −

Following methods are useful in handling QPixmap object −

The most common use of QPixmap is to display image on a label/button.

Example

The following example shows an image displayed on a QLabel by using the setPixmap() method.

The complete code is as follows −

The above code produces the following output −