Qt for Python Considerations#

API Changes#

One of the goals of PySide6 is to be API compatible with PyQt, with certain exceptions.

The latest considerations and known issues will be also reported in the wiki.

__hash__() function return value#

The hash value returned for the classes PySide6.QtCore.QDate, PySide6.QtCore.QDateTime, PySide6.QtCore.QTime, PySide6.QtCore.QUrl will be based on their string representations, thus objects with the same value will produce the same hash.

QString#

Methods and functions that change the contents of a QString argument were modified to receive an immutable Python Unicode (or str) and return another Python Unicode/str as the modified string.

The following methods had their return types modified this way:

Classes: QAbstractSpinBox, QDateTimeEdit, QDoubleSpinBox, QSpinBox, QValidator

  • fixup(string): string

  • validate(string, int): [QValidator.State, string, int]

Classes: QDoubleValidator, QIntValidator, QRegExpValidator

  • validate(string, int): [QValidator.State, string, int]

Class: QClipboard

  • text(string, QClipboard.Mode mode=QClipboard.Clipboard): [string, string]

Class: QFileDialog

Instead of getOpenFileNameAndFilter(), getOpenFileNamesAndFilter() and getSaveFileNameAndFilter() like PyQt does, PySide has modified the original methods to return a tuple.

  • getOpenFileName(QWidget parent=None, str caption=None, str dir=None, str filter=None, QFileDialog.Options options=0): [string, filter]

  • getOpenFileNames(QWidget parent=None, str caption=None, str dir=None, str filter=None, QFileDialog.Options options=0): [list(string), filter]

  • getSaveFileName(QWidget parent=None, str caption=None, str dir=None, str filter=None, QFileDialog.Options options=0): [string, filter]

Class: QWebPage

  • javaScriptPrompt(QWebFrame, string, string): [bool, string]

Classes: QFontMetrics and QFontMetricsF

They had two new methods added. Both take a string of one character and convert to a QChar (to call the C++ counterpart):

  • widthChar(string)

  • boundingRectChar(string)

QTextStream#

Inside this class some renames were applied to avoid clashes with native Python functions. They are: bin_(), hex_() and oct_(). The only modification was the addition of the ‘_’ character.

QVariant#

As QVariant was removed, any function expecting it can receive any Python object (None is an invalid QVariant). The same rule is valid when returning something: the returned QVariant will be converted to its original Python object type.

When a method expects a QVariant::Type the programmer can use a string (the type name) or the type itself.

qApp “macro”#

The C++ API of QtWidgets provides a macro called qApp that roughly expands to QtWidgets::QApplication->instance().

In PySide, we tried to create a macro-like experience. For that, the qApp variable was implemented as a normal variable that lives in the builtins. After importing PySide6, you can immediately use qApp.

As a useful shortcut for the action “create an application if it was not created”, we recommend:

qApp or QtWidgets.QApplication()

or if you want to check if there is one, simply use the truth value:

if qApp:
    # do something if an application was created
    pass

Comparing to None is also possible, but slightly over-specified.

Testing support#

For testing purposes, you can also get rid of the application by calling:

qApp.shutdown()

As for 5.14.2, this is currently an experimental feature that is not fully tested.

Embedding status#

In embedded mode, application objects that are pre-created in C++ don’t have a Python wrapper. The qApp variable is created together with a wrapped application. Therefore, qApp does not exist in that embedded mode. Please note that you always can use QtWidgets.QApplication.instance() instead.

Abandoned Alternative#

We also tried an alternative implementation with a qApp() function that was more pythonic and problem free, but many people liked the qApp macro better for its brevity, so here it is.

Rich Comparison#

There was a long-standing bug in the tp_richcompare implementation of PySide classes.

  • When a class did not implement it, the default implementation of object is used. This implements == and != like the is operator.

  • When a class implements only a single function like <, then the default implementation was disabled, and expressions like obj in sequence failed with NotImplemented.

This oversight was fixed in version 5.15.1 .

Qt for Python Features#

In Qt for Python, we begin for the first time to support a more pythonic user interface. With a special import statement, you can switch on features which replace certain aspects of the Python interpreter. This is done by an import statement right after the PySide6 import.

snake_case#

With the statement:

from __feature__ import snake_case

all methods in the current module are switched from camelCase to snake_case. A single upper case letter is replaced by an underscore and the lower case letter.

true_property#

With the statement:

from __feature__ import true_property

all getter and setter functions which are marked as a property in the Qt6 docs are replaced by Python property objects. Properties are also listed as such in the according QMetaObject of a class.

Example for both features#

Some Qt for Python snippet might read:

self.table.horizontalHeader().setSectionResizeMode(QHeaderView.Stretch)

With the above features selected, this reads:

self.table.horizontal_header().section_resize_mode = QHeaderView.Stretch

Additionally, properties can also be declared directly in Shiboken for non Qt-libraries, see property-declare.

More about features#

Detailed info about features can be found here: feature-why

Tools#

Qt for Python ships some Qt tools:

The New Python Enums#

The Motivation to use new Enums#

For a long time, there were just the Shiboken enums, which were modelled as exact as possible after the existing enums in Qt. These enums are small classes which also inherit from int.

Meanwhile, Python enums have been developed over the years. They have become a natural part of modern Python. The implementation is perfectly modelled after the needs of Python users. It is therefore just consequent to stop having two different enum implementations in the same application and instead to use the new Python implementation everywhere.

Existing Work#

The new enums beginning with PySide 6.3, replace the Shiboken enums with Python variants, which harmonize the builtin enums with the already existing QEnum “macro” shown in the QEnum/QFlag section.

Activating the New Enums#

The new approach to enum will be the default in PySide 6.4, but a preview is already built into PySide 6.3 with the environment variable: PYSIDE63_OPTION_PYTHON_ENUM=1 python3 <myscript> enables the new enums. In PySide 6.4, this flag is by default on, but it can be switched to the old Shiboken enums by setting the variable to 0.

This environment variable will stay until a general acceptance of the new enums has been reached and a fallback to the old implementation is no longer needed.

The Differences between old and new Enums#

Python enums and Shiboken enums are more or less compatible with each other. Tiny differences are in restrictions:

  • Python enums cannot inherit from each other, whereas Shiboken enums can

  • Python enums don’t allow undefined values, Shiboken enums do

  • Python enums always need exactly one argument, Shiboken enums have a default zero value

  • Python enums rarely inherit from int, Shiboken enums always do

More visible are the differences between flags, as shown in the following:

The Shiboken flag constructor example has been in PySide prior to 6.3:

flags = Qt.Alignment()
enum = Qt.AlignmentFlag

with enum shortcuts like

Qt.AlignLeft = Qt.AlignmentFlag.AlignLeft
Qt.AlignTop  = Qt.AlignmentFlag.AlignTop

In PySide 6.3, these shortcuts and flags no longer exist (officially). Instead, Python has an enum.Flags class which is a subclass of the enum.Enum class. But don’t be too scared, here comes the good news…

Doing a Smooth Transition from the Old Enums#

Changing all the enum code to suddenly use the new syntax is cumbersome and error-prone, because such necessary changes are not easy to find. Therefore a forgiveness mode was developed:

The forgiveness mode allows you to continue using the old constructs but translates them silently into the new ones. If you for example write

flags = Qt.Alignment()
enum = Qt.AlignLeft

item.setForeground(QColor(Qt.green))

flags_type = QPainter.RenderHints
flags = QPainter.RenderHints()

chart_view.setRenderHint(QPainter.Antialiasing)

you get in reality a construct that mimics the following code which is the recommended way of writing Flags and Enums:

flags = Qt.AlignmentFlag(0)
enum = Qt.AlignmentFlag.AlignLeft

item.setForeground(QColor(Qt.GlobalColor.green))

flags_type = QPainter.RenderHint
flags = QPainter.RenderHint(0)

chart_view.setRenderHint(QPainter.RenderHint.Antialiasing)

This has the effect that you can initially ignore the difference between old and new enums, as long as the new enums are properties of classes. (This does not work on global enums which don’t have a class, see Limitations below.)

Forgiveness Mode and Type Hints#

When you inspect for instance QtCore.pyi, you will only find the new enums, although the old ones are still allowed. Also, line completion will only work with the new constructs and never propose the old ones.

The reason to implement forgiveness mode this way was

  • to make the transition as smooth as possible, but

  • to encourage people to use the new enums whenever new code is written.

So you can continue to write:

self.text.setAlignment(Qt.AlignCenter)

but this construct is used and recommended for the future:

self.text.setAlignment(Qt.AlignmentFlag.AlignCenter)

Limitations:#

The forgiveness mode works very well whenever the enum class is embedded in a normal PySide class. But there are a few global enums, where especially the QtMsgType is a problem:

t = QtMsgType.QtDebugMsg

cannot be written in the shortcut form

t = QtDebugMsg

because there is no surrounding PySide class that provides the forgiving mode implementation. Typically, the needed changes are easily found because they often occur in an import statement.