Support for QVariant — PyQt 5.6 Reference Guide

# Support for `QVariant`

PyQt4 implements two APIs for `QVariant`. v1 (the default for Python v2) exposes the `QVariant` class to Python and requires applications to explicitly convert a `QVariant` to the actual value. v2 (the default for Python v3) does not expose the `QVariant` class to Python and automatically converts a `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++ `QVariant` and the corresponding Python object. For example a `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 `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 `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++ `QVariant` to a Python object and to return a wrapped Python `QVariant` instead - behaviour similar to PyQt4’s v1 API - by calling the `sip.enableautoconversion()` function.

The actual value of a wrapped Python `QVariant` is obtained by calling its `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.