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CAVEATS.md

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Caveats

There are cases where Qt.py is not handling incompatibility issues.



Tests

Code blocks in this document are automatically tested at each commit before being accepted into the project. In order for your code to run successfully, follow these guidelines.

  1. Each caveat MUST contain (1) a header, (2) description, (3) one or more examples and (4, optional) a solution.
  2. Each caveat MUST have a header prefixed with four hashtags, e.g. #### My Heading.
  3. Each example MAY NOT use more than one (1) binding at a time, e.g. both PyQt5 and PySide.
  4. Each example MUST visualise return value and any exceptions thrown.
  5. An example MUST reside under a heading, e.g. #### My Heading
  6. The first line of each example MUST be # MyBinding, where MyBinding is the binding you intend to test with, such as PySide or PyQt4.
  7. Examples MAY indicate either Python 2 or 3 as # MyBinding, Python2
  8. Examples MUST be in doctest format. See other caveats for samples.
  9. Examples MUST import Qt (where appropriate), NOT e.g. import PyQt5.
  10. Examples MAY include untested in which case the continuous integration mechanism will look the other way, e.g. # PyQt4, untested



QtGui.QAbstractItemModel.createIndex

In PySide, somehow the last argument (the id) is allowed to be negative and is maintained. While in PyQt4 it gets coerced into an undefined unsigned value.

# PySide
>>> from Qt import QtGui
>>> model = QtGui.QStandardItemModel()
>>> index = model.createIndex(0, 0, -1)
>>> int(index.internalId()) == -1
True
# PyQt4
>>> from Qt import QtGui
>>> model = QtGui.QStandardItemModel()
>>> index = model.createIndex(0, 0, -1)
>>> int(index.internalId()) == 18446744073709551615
True
Usecase

I had been using the id as an index into a list. But the unexpected return value from PyQt4 broke it by being invalid. The workaround was to always check that the returned id was between 0 and the max size I expect.

– @justinfx




QtCore.QItemSelection

PySide has the QItemSelection.isEmpty and QItemSelection.empty attributes while PyQt4 only has the QItemSelection.isEmpty attribute.

# PySide2
>>> from Qt import QtCore
>>> func = QtCore.QItemSelection.isEmpty
>>> func = QtCore.QItemSelection.empty
# PyQt5
>>> from Qt import QtCore
>>> func = QtCore.QItemSelection.isEmpty
>>> func = QtCore.QItemSelection.empty
Traceback (most recent call last):
...
AttributeError: type object 'QItemSelection' has no attribute 'empty'
Workaround

They both support the len(selection) operation.

# PyQt4
>>> from Qt import QtCore
>>> selection = QtCore.QItemSelection()
>>> len(selection)
0
# PySide
>>> from Qt import QtCore
>>> selection = QtCore.QItemSelection()
>>> len(selection)
0



QtCore.Slot

PySide allows for a result=None keyword param to set the return type. PyQt4 crashes:

# PySide
>>> from Qt import QtCore, QtWidgets
>>> slot = QtCore.Slot(QtWidgets.QWidget, result=None)
# PyQt4, Python2
>>> from Qt import QtCore, QtWidgets
>>> slot = QtCore.Slot(QtWidgets.QWidget)
>>> slot = QtCore.Slot(QtWidgets.QWidget, result=None)
Traceback (most recent call last):
...
TypeError: string or ASCII unicode expected not 'NoneType'
# PyQt4, Python3
>>> from Qt import QtCore, QtWidgets
>>> slot = QtCore.Slot(QtWidgets.QWidget)
>>> slot = QtCore.Slot(QtWidgets.QWidget, result=None)
Traceback (most recent call last):
...
TypeError: bytes or ASCII string expected not 'NoneType'



QtWidgets.QAction.triggered

PySide cannot accept any arguments. In PyQt4, QAction.triggered signal requires a bool arg.

# PySide
>>> from Qt import QtCore, QtWidgets
>>> obj = QtCore.QObject()
>>> action = QtWidgets.QAction(obj)
>>> action.triggered.emit()  # Note the return value (!)
True
>>> action.triggered.emit(True)
Traceback (most recent call last):
...
TypeError: triggered() only accepts 0 arguments, 2 given!
# PyQt4
>>> from Qt import QtCore, QtWidgets
>>> obj = QtCore.QObject()
>>> action = QtWidgets.QAction(obj)
>>> action.triggered.emit(True)
>>> action.triggered.emit()
Traceback (most recent call last):
...
TypeError: QAction.triggered[bool] signal has 1 argument(s) but 0 provided



QtGui.QRegExpValidator

Affects Version
PyQt4 <= 4.8.4

In PySide, the constructor for QtGui.QRegExpValidator() can just take a QRegExp instance, and that is all.

In PyQt4 you are required to pass some form of a parent argument, otherwise you get a TypeError:

# PySide, untested
>>> from Qt import QtCore, QtGui
>>> regex = QtCore.QRegExp("\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}")
>>> validator = QtGui.QRegExpValidator(regex)
>>> validator = QtGui.QRegExpValidator(regex, None)
Traceback (most recent call last):
...
TypeError: ...
# PyQt4, untested
>>> from Qt import QtCore, QtGui
>>> regex = QtCore.QRegExp("\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}")
>>> validator = QtGui.QRegExpValidator(regex, None)
>>> validator = QtGui.QRegExpValidator(regex)
Traceback (most recent call last):
...
TypeError: ...



QtWidgets.QHeaderView.setResizeMode

setResizeMode was renamed setSectionResizeMode in Qt 5.

# PySide2
>>> from Qt import QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> view = QtWidgets.QTreeWidget()
>>> header = view.header()
>>> header.setResizeMode(QtWidgets.QHeaderView.Fixed)
Traceback (most recent call last):
...
AttributeError: 'PySide2.QtWidgets.QHeaderView' object has no attribute 'setResizeMode'
# PySide
>>> from Qt import QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> view = QtWidgets.QTreeWidget()
>>> header = view.header()
>>> header.setSectionResizeMode(QtWidgets.QHeaderView.Fixed)
Traceback (most recent call last):
...
AttributeError: 'PySide.QtGui.QHeaderView' object has no attribute 'setSectionResizeMode'
Workaround

Use compatibility wrapper.

# PySide2
>>> from Qt import QtWidgets, QtCompat
>>> app = QtWidgets.QApplication(sys.argv)
>>> view = QtWidgets.QTreeWidget()
>>> header = view.header()
>>> QtCompat.setSectionResizeMode(header, QtWidgets.QHeaderView.Fixed)

Or a conditional.

# PyQt5
>>> from Qt import QtWidgets, __binding__
>>> app = QtWidgets.QApplication(sys.argv)
>>> view = QtWidgets.QTreeWidget()
>>> header = view.header()
>>> if __binding__ in ("PyQt4", "PySide"):
...   header.setResizeMode(QtWidgets.QHeaderView.Fixed)
... else:
...   header.setSectionResizeMode(QtWidgets.QHeaderView.Fixed)

QtWidgets.qApp

qApp is not included in Qt.py due to the way Qt keeps this up to date with the currently active QApplication.

Qt implicitly updates this variable through monkey patching whenever a new QApplication is instantiated. This means that our variable quickly goes out of date and is not updated at the same time.

# PySide2
>>> from Qt import QtWidgets
>>> "qApp" in dir(QtWidgets)
False
Workaround

Use QApplication.instance() instead.

Technically, there is no difference between the two, apart from more characters to type.

# PySide2
>>> from Qt import QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> app == QtWidgets.QApplication.instance()
True

QtCompat.wrapInstance

QtCompat.wrapInstance differs across sip and shiboken in subtle ways.

# PySide2
>>> from Qt import QtCompat, QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> button = QtWidgets.QPushButton("Hello world")
>>> button.setObjectName("MySpecialButton")
>>> pointer = QtCompat.getCppPointer(button)
>>> widget = QtCompat.wrapInstance(long(pointer))
>>> assert isinstance(widget, QtWidgets.QWidget), widget
>>> assert widget.objectName() == button.objectName()
>>> widget == button
False
# PyQt5
>>> from Qt import QtCompat, QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> button = QtWidgets.QPushButton("Hello world")
>>> button.setObjectName("MySpecialButton")
>>> pointer = QtCompat.getCppPointer(button)
>>> widget = QtCompat.wrapInstance(long(pointer))
>>> assert isinstance(widget, QtWidgets.QWidget), widget
>>> assert widget.objectName() == button.objectName()
>>> widget == button
True

Note the False for PySide2 and True for PyQt5.