Qt for Python Considerations

API Changes

One of the goals of PySide2 is to be API compatible with PyQt5, 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 PySide2.QtCore.QDate, PySide2.QtCore.QDateTime, PySide2.QtCore.QTime, PySide2.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 PySide2, 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.