.. _pysideapi2:

Considerations
==============

API Changes
-----------

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

The latest considerations and known issues will be also reported
in the `wiki <https://wiki.qt.io/Qt_for_Python/Considerations>`_.

__hash__() function return value
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The hash value returned for the classes :class:`PySide6.QtCore.QDate`,
:class:`PySide6.QtCore.QDateTime`, :class:`PySide6.QtCore.QTime`, :class:`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 .


Features
--------

In |project|, 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:

.. code-block:: python

    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:

.. code-block:: python

    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 |project| snippet might read:

.. code-block:: python

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

With the above features selected, this reads:

.. code-block:: python

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

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

More about features
~~~~~~~~~~~~~~~~~~~

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

Tools
~~~~~

|project| ships some Qt tools:

* ``pyside6-rcc``: Qt Resource Compiler. This is a command line tool
  that compiles ``.qrc`` files containing binary data, for example images,
  into executable Python code (see :ref:`using_qrc_files`).
* ``pyside6-uic``: Qt User Interface Compiler. This is a command line tool
  that compiles ``.ui`` files containing designs of Qt Widget-based forms
  into executable Python code (see :ref:`using_ui_files`).
* ``pyside6-assistant``: Qt Help Viewer. This is a graphical tool that can
  be used to view Qt documentation from Qt Compressed Help files (``.qhc``).
  Currently, only the binary without documentation sets is shipped to reduce the
  wheel size. For building the documentation, see :ref:`building_documentation`.
* ``pyside6-designer``: Qt User Interface Designer. This is a graphical tool
  to create designs of Qt Widget-based forms and use custom widgets
  (see :ref:`using_ui_files`, :ref:`designer_custom_widgets`).


.. _NewEnumSystem:

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 :ref:`QEnum` 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.
