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.. _ref-qvariant:
Support for :class:`~PyQt5.QtCore.QVariant`
===========================================
PyQt4 implements two APIs for :class:`~PyQt5.QtCore.QVariant`. v1 (the default
for Python v2) exposes the :class:`~PyQt5.QtCore.QVariant` class to Python and
requires applications to explicitly convert a :class:`~PyQt5.QtCore.QVariant`
to the actual value. v2 (the default for Python v3) does not expose the
:class:`~PyQt5.QtCore.QVariant` class to Python and automatically converts a
:class:`~PyQt5.QtCore.QVariant` to the actual value. While this is usually the
best thing to do, it does raise problems of its own:
- Information is lost when converting between a C++
:class:`~PyQt5.QtCore.QVariant` and the corresponding Python object. For
example a :class:`~PyQt5.QtCore.QVariant` distinguishes between signed and
unsigned integers but Python doesn't. Normally this doesn't matter but some
applications may need to make the distinction.
- There is no obvious way to represent a null :class:`~PyQt5.QtCore.QVariant`
as a standard Python object. PyQt4 introduced the ``QPyNullVariant`` class
to address this problem.
Multiple APIs are intended to help manage an application's use of an old API to
a newer, incompatible API. They cannot be used to temporarily change the
behaviour - modules that rely on different API versions cannot be used in the
same application.
In PyQt5 the implementation of :class:`~PyQt5.QtCore.QVariant` is different to
those of PyQt4. By default the behaviour is the same as PyQt4's v2 API.
However it is possible to temporarily suppress the automatic conversion of a
C++ :class:`~PyQt5.QtCore.QVariant` to a Python object and to return a wrapped
Python :class:`~PyQt5.QtCore.QVariant` instead - behaviour similar to PyQt4's
v1 API - by calling the :func:`sip.enableautoconversion` function.
The actual value of a wrapped Python :class:`~PyQt5.QtCore.QVariant` is
obtained by calling its :meth:`~PyQt5.QtCore.QVariant.value` method. (Note
that in PyQt4's v1 API this method is called ``toPyObject()``.)
PyQt5 does not support the ``QPyNullVariant`` class as it is no longer needed.
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