Using Qt Designer — PyQt 4.9.1 Reference Guide

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# Using Qt Designer

Qt Designer is the Qt tool for designing and building graphical user interfaces. It allows you to design widgets, dialogs or complete main windows using on-screen forms and a simple drag-and-drop interface. It has the ability to preview your designs to ensure they work as you intended, and to allow you to prototype them with your users, before you have to write any code.

Qt Designer uses XML `.ui` files to store designs and does not generate any code itself. Qt includes the `uic` utility that generates the C++ code that creates the user interface. Qt also includes the `QUiLoader` class that allows an application to load a `.ui` file and to create the corresponding user interface dynamically.

PyQt does not wrap the `QUiLoader` class but instead includes the `uic` Python module. Like `QUiLoader` this module can load `.ui` files to create a user interface dynamically. Like the uic utility it can also generate the Python code that will create the user interface. PyQt’s pyuic4 utility is a command line interface to the `uic` module. Both are described in detail in the following sections.

## Using the Generated Code

The code that is generated has an identical structure to that generated by Qt’s `uic` and can be used in the same way.

The code is structured as a single class that is derived from the Python `object` type. The name of the class is the name of the toplevel object set in Designer with `Ui_` prepended. (In the C++ version the class is defined in the `Ui` namespace.) We refer to this class as the form class.

The class contains a method called `setupUi()`. This takes a single argument which is the widget in which the user interface is created. The type of this argument (typically `QDialog`, `QWidget` or `QMainWindow`) is set in Designer. We refer to this type as the Qt base class.

In the following examples we assume that a `.ui` file has been created containing a dialog and the name of the `QDialog` object is `ImageDialog`. We also assume that the name of the file containing the generated Python code is `ui_imagedialog.py`. The generated code can then be used in a number of ways.

The first example shows the direct approach where we simply create a simple application to create the dialog:

    import sys
    from PyQt4.QtGui import QApplication, QDialog
    from ui_imagedialog import Ui_ImageDialog

    app = QApplication(sys.argv)
    window = QDialog()
    ui = Ui_ImageDialog()
    ui.setupUi(window)

    window.show()
    sys.exit(app.exec_())

The second example shows the single inheritance approach where we sub-class `QDialog` and set up the user interface in the `__init__()` method:

    from PyQt4.QtGui import QDialog
    from ui_imagedialog import Ui_ImageDialog

    class ImageDialog(QDialog):
        def __init__(self):
            QDialog.__init__(self)

            # Set up the user interface from Designer.
            self.ui = Ui_ImageDialog()
            self.ui.setupUi(self)

            # Make some local modifications.
            self.ui.colorDepthCombo.addItem("2 colors (1 bit per pixel)")

            # Connect up the buttons.
            self.ui.okButton.clicked.connect(self.accept)
            self.ui.cancelButton.clicked.connect(self.reject)

The third example shows the multiple inheritance approach:

    from PyQt4.QtGui import QDialog
    from ui_imagedialog import Ui_ImageDialog

    class ImageDialog(QDialog, Ui_ImageDialog):
        def __init__(self):
            QDialog.__init__(self)

            # Set up the user interface from Designer.
            self.setupUi(self)

            # Make some local modifications.
            self.colorDepthCombo.addItem("2 colors (1 bit per pixel)")

            # Connect up the buttons.
            self.ui.okButton.clicked.connect(self.accept)
            self.ui.cancelButton.clicked.connect(self.reject)

It is also possible to use the same approach used in PyQt v3. This is shown in the final example:

    from ui_imagedialog import ImageDialog

    class MyImageDialog(ImageDialog):
        def __init__(self):
            ImageDialog.__init__(self)

            # Make some local modifications.
            self.colorDepthCombo.addItem("2 colors (1 bit per pixel)")

            # Connect up the buttons.
            self.ui.okButton.clicked.connect(self.accept)
            self.ui.cancelButton.clicked.connect(self.reject)

For a full description see the Qt Designer Manual in the Qt Documentation.

## The `uic` Module

The `uic` module contains the following functions and objects.

`PyQt4.uic.` `widgetPluginPath`

  The list of the directories that are searched for widget plugins. Initially it contains the name of the directory that contains the widget plugins included with PyQt.

`PyQt4.uic.` `compileUi`(uifile, pyfile[, execute=False[, indent=4[, pyqt3_wrapper=False[, from_imports=False]]]])

  Generate a Python module that will create a user interface from a Qt Designer `.ui` file.

  Parameters: | uifile – the file name or file-like object containing the `.ui` file. pyfile – the file-like object to which the generated Python code will be written to. execute – is optionally set if a small amount of additional code is to be generated that will display the user interface if the code is run as a standalone application. indent – the optional number of spaces used for indentation in the generated code. If it is zero then a tab character is used instead. pyqt3_wrapper – is optionally set if a small wrapper is to be generated that allows the generated code to be used as it is by PyQt v3 applications. from_imports – is optionally set to generate import statements that are relative to `'.'`. At the moment this only applies to the import of resource modules.

`PyQt4.uic.` `compileUiDir`(dir[, recurse=False[, map=None[, **compileUi_args]]])

  Create Python modules from Qt Designer `.ui` files in a directory or directory tree.

  Parameters: | dir – the name of the directory to scan for files whose name ends with `.ui`. By default the generated Python module is created in the same directory ending with `.py`. recurse – is optionally set if any sub-directories should be scanned. map – an optional callable that is passed the name of the directory containing the `.ui` file and the name of the Python module that will be created. The callable should return a tuple of the name of the directory in which the Python module will be created and the (possibly modified) name of the module. compileUi_args – are any additional keyword arguments that are passed to `compileUi()` that is called to create each Python module.

`PyQt4.uic.` `loadUiType`(uifile[, from_imports=False])

  Load a Qt Designer `.ui` file and return a tuple of the generated form class and the Qt base class. These can then be used to create any number of instances of the user interface without having to parse the `.ui` file more than once.

  Parameters: | uifile – the file name or file-like object containing the `.ui` file. from_imports – is optionally set to use import statements that are relative to `'.'`. At the moment this only applies to the import of resource modules.
  Return type: | the form class and the Qt base class.

`PyQt4.uic.` `loadUi`(uifile[, baseinstance=None])

  Load a Qt Designer `.ui` file and returns an instance of the user interface.

  Parameters: | uifile – the file name or file-like object containing the `.ui` file. baseinstance – the optional instance of the Qt base class. If specified then the user interface is created in it. Otherwise a new instance of the base class is automatically created.
  Return type: | the `QWidget` sub-class that implements the user interface.

## pyuic4

The pyuic4 utility is a command line interface to the `uic` module. The command has the following syntax:

    pyuic4 [options] .ui-file

The full set of command line options is:

`-h````,` `--help```

  A help message is written to `stdout`.

`--version```

  The version number is written to `stdout`.

`-i``<N>``,` `--indent``<N>`

  The Python code is generated using an indentation of `<N>` spaces. If `<N>` is 0 then a tab is used. The default is 4.

`-o``<FILE>``,` `--output``<FILE>`

  The Python code generated is written to the file `<FILE>`.

`-p````,` `--preview```

  The GUI is created dynamically and displayed. No Python code is generated.

`-w````,` `--pyqt3-wrapper```

  The generated Python code includes a small wrapper that allows the GUI to be used in the same way as it is used in PyQt v3.

`-x````,` `--execute```

  The generated Python code includes a small amount of additional code that creates and displays the GUI when it is executes as a standalone application.

`--from-imports```

  Resource modules are imported using `from . import` rather than a simple `import`.

Note that code generated by pyuic4 is not guaranteed to be compatible with earlier versions of PyQt. However, it is guaranteed to be compatible with later versions. If you have no control over the version of PyQt the users of your application are using then you should run pyuic4, or call `compileUi()`, as part of your installation process. Another alternative would be to distribute the `.ui` files (perhaps as part of a resource file) and have your application load them dynamically.

## Writing Qt Designer Plugins

Qt Designer can be extended by writing plugins. Normally this is done using C++ but PyQt also allows you to write plugins in Python. Most of the time a plugin is used to expose a custom widget to Designer so that it appears in Designer’s widget box just like any other widget. It is possibe to change the widget’s properties and to connect its signals and slots.

It is also possible to add new functionality to Designer. See the Qt documentation for the full details. Here we will concentrate on describing how to write custom widgets in Python.

The process of integrating Python custom widgets with Designer is very similar to that used with widget written using C++. However, there are particular issues that have to be addressed.

- Designer needs to have a C++ plugin that conforms to the interface defined by the `QDesignerCustomWidgetInterface` class. (If the plugin exposes more than one custom widget then it must conform to the interface defined by the `QDesignerCustomWidgetCollectionInterface` class.) In addition the plugin class must sub-class `QObject` as well as the interface class. PyQt does not allow Python classes to be sub-classed from more than one Qt class.
- Designer can only connect Qt signals and slots. It has no understanding of Python signals or callables.
- Designer can only edit Qt properties that represent C++ types. It has no understanding of Python attributes or Python types.

PyQt provides the following components and features to resolve these issues as simply as possible.

- PyQt’s QtDesigner module includes additional classes (all of which have a `QPy` prefix) that are already sub-classed from the necessary Qt classes. This avoids the need to sub-class from more than one Qt class in Python. For example, where a C++ custom widget plugin would sub-class from `QObject` and `QDesignerCustomWidgetInterface`, a Python custom widget plugin would instead sub-class from `QPyDesignerCustomWidgetPlugin`.

- PyQt installs a C++ plugin in Designer’s plugin directory. It conforms to the interface defined by the `QDesignerCustomWidgetCollectionInterface` class. It searches a configurable set of directories looking for Python plugins that implement a class sub-classed from `QPyDesignerCustomWidgetPlugin`. Each class that is found is instantiated and the instance created is added to the custom widget collection.

  The `PYQTDESIGNERPATH` environment variable specifies the set of directories to search for plugins. Directory names are separated by a path separator (a semi-colon on Windows and a colon on other platforms). If a directory name is empty (ie. there are consecutive path separators or a leading or trailing path separator) then a set of default directories is automatically inserted at that point. The default directories are the `python` subdirectory of each directory that Designer searches for its own plugins. If the environment variable is not set then only the default directories are searched. If a file’s basename does not end with `plugin` then it is ignored.

- A Python custom widget may define new Qt signals using `pyqtSignal()`.

- A Python method may be defined as a new Qt slot by using the `pyqtSlot()` decorator.

- A new Qt property may be defined using the `pyqtProperty()` function.

Note that the ability to define new Qt signals, slots and properties from Python is potentially useful to plugins conforming to any plugin interface and not just that used by Designer.

For a simple but complete and fully documented example of a custom widget that defines new Qt signals, slots and properties, and its plugin, look in the `examples/designer/plugins` directory of the PyQt source package. The `widgets` subdirectory contains the `pydemo.py` custom widget and the `python` subdirectory contains its `pydemoplugin.py` plugin.

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