PSEP:101
Title:Adopt PyQt's API 2 for PySide
Version:1
Last-Modified:15-Aug-2011
Author:Mark Summerfield <mark at qtrac.eu>
Status:Final
Type:Standards Track
Content-Type:text/x-rst
Created:14-Apr-2010
Post-History:14-Apr-2010, 15-Apr-2010, 16-Apr-2010, 20-Apr-2010, 22-Apr-2010, 18-May-2010

Contents

Abstract

PyQt4 provides two different (and incompatible) APIs. API 1 is the original API and the one supported by PySide. API 2 is a new Python 3-specific API that is much more Pythonic. This PSEP proposes that PySide adopt PyQt4's API 2 for PySide.

Rationale

PyQt4 provides two different (and incompatbile) APIs [1]. API 1 is useful for those using PyQt4 to prototype C++/Qt programs since it is very close to the C++/Qt API. However, for those who want to create Python Qt programs in their own right, and especially for existing Python programmers, API 1 is cumbersome and un-Pythonic when it comes to handling QStrings and QVariants, both of which are used a lot.

If PySide were to support API 2, it will make PySide much more attractive to Python 3 programmers.

Also, supporting API 2 will mean that existing Python 3/PyQt4 programs that use API 2 by default will be able to switch to PySide.

One key purpose of API 2 is to avoid the need for Python programmers to have to worry or even know about QString or QVariant. For Python programmers str is the string they are used to so translation to/from QString should be transparent and automatic. Similarly, why should Python programmers have to know about QVariant when that is a C++ workaround for C++'s lack of support for duck typing etc.? Again, translation to/from QVariant should be transparent and automatic. API 2 achieves this and eliminates QString and (to some extent) QVariant.

API 1 vs. API 2

No QString and almost no QVariant

The key difference between the APIs is that API 2 does not make QString or (to some extent) QVariant available to programmers.

This means using Python's native string types for strings (e.g., unicode for Python 2 and str for Python 3).

PyQt's approach to dealing with QVariants will be mimicked: Python objects are automatically converted to and from QVariants wherever QVariants must be passed as arguments and when QVariants are returned.

By supporting automatic conversion to and from QVariants, any Python object (including dicts, lists, and tuples) can be used wherever a QVariant is expected, so most programmers most of the time won't have to know or care about QVariant at all.

For QString, API 2 goes beyond simply replacing QString with str (or unicode on Python 2). In particular, API 2 also automatically converts QChar and QStringRef to and from strs (or unicodes on Python 2). It also converts QStringList to and from a Python list of strs (list of unicodes on Python 2). (PyQt4's API 2 does not implement QLatin1Char, QLatin1String, or QStringMatcher.)

Native C++/Qt QStrings are mutable, so PyQt4's API 2 has changed the signatures of some of the affected functions to return a str (or a tuple including a str) when in the C++/Qt API a QString parameter would have been modified.

In addition, API 2 has added two methods to QFontMetrics and QFontMetricsF, widthChar() and boundingRectChar(), which take a str of length one and call QFontMetrics::width() and QFontMetrics::boundingRect() with a QChar argument. These were added because the width of a single character may be different from that of a string that contains a single character, even if the characters are the same.

If the need arises during the implementation phase, the QVariant class may be added to the API so that programmers can pass a QVariant explicitly if they need to. If this is done, to support the creation of QVariants, a set of static functions named after the types in QVariant.Type will need to be implemented, e.g.:

QVariant.fromUInt(pythonObject) -> QVariant
QVariant.fromInt(pythonObject) -> QVariant

The benefit of API 2 in this regard is that programmers never have to explicitly convert between QString and str, so programmers never get caught out accidentally trying to use a QString method on a str or vice versa. For the Qt APIs that accept or return QVariants, API 2 allows programmers to pass any Python object and when a QVariant is returned from Qt it is automatically converted to its original Python type (with an invalid QVariant mapped to None).

Changes to Other Types

API 2 also affects some other types, as follows.

Where QByteArrays are expected, bytes objects can be used, but unlike QString, QByteArray has been retained to ease porting from API 1, so in API 2 byte sequences can be represented by QByteArray, bytes, and bytearray objects.

QDate, QDateTime, QTime, and QUrl's __hash__() methods return a string representation so that identical dates (and identical date/times or times or URLs) will have identical hash values.

QTextStream's bin(), hex(), and oct() functions have been renamed bin_(), hex_(), and oct_(), to avoid conflicting with Python's built-ins of the same names.

Support for Keyword Arguments

PyQt 4.7 adds support for keyword arguments which is very convenient and much more Pythonic.

This change represents an extension to the PyQt APIs that a compatible PySide ought to implement.

One problem with this is that while changes to argument names don't affect C++/Qt, they would break the Python API. Furthermore, since changing argument names is harmless in C++/Qt, such changes do take place between Qt versions. This means that for PySide, sensible names must be used in the first place---and stuck to. Naturally, for PyQt compatibility, PySide ought to use the same names as PyQt.

PyQt does not integrate Python and Qt's property systems, but it does allow Qt properties to be set when an object is constructed using keyword arguments (i.e., where the keyword is the name of a Qt property). The same functionality will be implemented in PySide. Also, PyQt provides the pyqtConfigure() method for all objects that have Qt properties: this method can be called at any time and the object's Qt properties set by passing it keyword arguments. However, since essentially the same functionality is already provided by QObject.setProperty() and .property() methods, pyqtConfigure() will not be implemented as part of this PSEP.

Compatibility

The proposed API 2 is incompatible with API 1, but since PySide is not yet widely used and hasn't reached version 1.0.0, it seems reasonable to break compatibility for the sake of getting PySide "right". Furthermore, if API 2 is adopted it means that PySide programs will have the same API whether Python 2 or Python 3 is used which will make it much easier for those who start out with PySide and Python 2 and later on port to Python 3---something that isn't so easy with PyQt.

References

[1]PyQt v4 Reference Manual: Potential Incompatibilities section, Selecting Incompatible APIs section, Support for Keyword Arguments section, and Support for Qt Properties section (http://www.riverbankcomputing.co.uk/static/Docs/PyQt4/pyqt4ref.html)

Contributors

Richard Dale and Detlev Offenbach pointed out that QByteArray had been retained in API 2.

Richard Dale and Lauro Moura gave suggestions regarding QVariant.

Hugo Lima proposed choosing one of PyQt's alternative APIs and implementing only that for both Python 2 and Python 3 PySide versions.

The section of Exceptions was dropped as a result of feedback from Matti Airas; Matti also added to the discussion in numerous ways.

Detlev Offenbach also contributed to the discussion on Compatibility.

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