PSEP: 101

Title: Adopt PyQt's API 2 for PySide

Version: $Revision$

Last-Modified: $Date$

Author: Mark Summerfield <mark@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

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 [#PyQt-APIs]_. 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

==========

.. [#PyQt-APIs] 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.

Copyright

=========

This document has been placed in the public domain.