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.
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 theis
operator.When a class implements only a single function like
<
, then the default implementation was disabled, and expressions likeobj in sequence
failed withNotImplemented
.
This oversight was fixed in version 5.15.1 .
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