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diff --git a/sources/pyside2/doc/tutorials/portingguide/index.rst b/sources/pyside2/doc/tutorials/portingguide/index.rst new file mode 100644 index 000000000..8fd4c431a --- /dev/null +++ b/sources/pyside2/doc/tutorials/portingguide/index.rst @@ -0,0 +1,194 @@ +Porting a C++ Application to Python +************************************* + +Qt for Python lets you use Qt APIs in a Python application. +So the next question is: What does it take to port an +existing C++ application? Try porting a Qt C++ application +to Python to understand this. + +Before you start, ensure that all the prerequisites for +Qt for Python are met. See +:doc:`Getting Started <../../gettingstarted>` for more +information. In addition, familiarize yourself with the +basic differences between Qt in C++ and in Python. + +Basic differences +================== + +This section highlights some of the basic differences +between C++ and Python, and how Qt differs between these +two contexts. + +C++ vs Python +-------------- + +* In the interest of code reuse, both C++ and Python + provide ways for one file of code to use facilities + provided by another. In C++, this is done using the + ``#include`` directive to access the API definition of + the reused code. The Python equivalent is an ``import`` + statement. +* The constructor of a C++ class shares the name of its + class and automatically calls the constructor of any + base-classes (in a predefined order) before it runs. + In Python, the ``__init__()`` method is the constructor + of the class, and it can explicitly call base-class + constructors in any order. +* C++ uses the keyword, ``this``, to implicitly refer to + the current object. In python, you need to explicitly + mention the current object as the first parameter + to each instance method of the class; it is conventionally + named ``self``. +* And more importantly, forget about curly braces, {}, and + semi-colon, ;. +* Precede variable definitions with the ``global`` keyword, + only if they need global scope. + +.. code:: python + + var = None + def func(key, value = None): + """Does stuff with a key and an optional value. + + If value is omitted or None, the value from func()'s + last call is reused. + """ + global var + if value is None: + if var is None: + raise ValueError("Must pass a value on first call", key, value) + value = var + else: + var = value + doStuff(key, value) + +In this example, ``func()`` would treat ``var`` as a local +name without the ``global`` statement. This would lead to +a ``NameError`` in the ``value is None`` handling, on +accessing ``var``. For more information about this, see + `Python refernce documentation <python refdoc>`_. + +.. _python refdoc: https://docs.python.org/3/reference/simple_stmts.html#the-global-statement + +.. tip:: Python being an interpreted language, most often + the easiest way is to try your idea in the interperter. + You could call the ``help()`` function in the + interpreter on any built-in function or keyword in + Python. For example, a call to ``help(import)`` should + provide documentation about the ``import`` statment + +Last but not the least, try out a few examples to +familiarize yourself with the Python coding style and +follow the guidelines outlined in the +`PEP8 - Style Guide <pep8>`_. + +.. _pep8: <https://www.python.org/dev/peps/pep-0008/#naming-conventions> + +.. code-block:: python + + import sys + + from PySide2.QtWidgets import QApplication, QLabel + + app = QApplication(sys.argv) + label = QLabel("Hello World") + label.show() + sys.exit(app.exec_()) + +.. note:: Qt provides classes that are meant to manage + the application-specific requirements depending on + whether the application is console-only + (QCoreApplication), GUI with QtWidgets (QApplication), + or GUI without QtWidgets (QGuiApplication). These + classes load necessary plugins, such as the GUI + libraries required by an application. In this case, it is + QApplication that is initialized first as the application + has a GUI with QtWidgets. + +Qt in the C++ and Python context +--------------------------------- + +Qt behaves the same irrespective of whether it is used +in a C++ or a Python application. Considering that C++ +and Python use different language semantics, some +differences between the two variants of Qt are inevitable. +Here are a few important ones that you must be aware of: + +* **Qt Properties**: ``Q_PROPERTY`` macros are used in C++ to add a + public member variable with getter and setter functions. Python's + alternative for this is the ``@property`` decorator before the + getter and setter function definitions. +* **Qt Signals and Slots**: Qt offers a unique callback mechanism, + where a signal is emitted to notify the occurrence of an event, so + that slots connected to this signal can react to it. In C++, + the class definition must define the slots under the + ``public Q_SLOTS:`` and signals under ``Q_SIGNALS:`` + access specifier. You connect these two using one of the + several variants of the QObject::connect() function. Python's + equivalent for this is the `@Slot`` decorator just before the + function definition. This is necessary to register the slots + with the QtMetaObject. +* **QString, QVariant, and other types**: + - Qt for Python does not provide access to QString and + QVariant. You must use Python's native types instead. + - QChar and QStringRef are represented as Python strings, + and QStringList is converted to a list of strings. + - QDate, QDateTime, QTime, and QUrl's __hash__() methods + return a string representation so that identical dates + (and identical date/times or times or URLs) have + identical hash values. + - QTextStream's bin(), hex(), and oct() functions are + renamed to bin_(), hex_(), and oct_() respectively. This + should avoid name conflicts with Python's built-in + functions. +* **QByteArray**: A QByteArray is treated as a list of + bytes without encoding. The equivalent type in Python + varies; Python 2 uses "str" type, whereas Python 3 uses + "bytes". To avoid confusion, a QString is represented as + an encoded human readable string, which means it is + a "unicode" object in Python 2, and a "str" in Python 3. + +Here is the improved version of the Hello World example, +demonstrating some of these differences: + +.. literalinclude:: hello_world_ex.py + :linenos: + :lines: 40- + +.. note:: The ``if`` block is just a good practice when + developing a Python application. It lets the Python file + behave differently depending on whether it is imported + as a module in another file or run directly. The + ``__name__`` variable will have different values in + these two scenarios. It is ``__main__`` when the file is + run directly, and the module's file name + (``hello_world_ex`` in this case) when imported as a + module. In the later case, everything defined in the + module except the ``if`` block is available to the + importing file. + +Notice that the QPushButton's ``clicked`` signal is +connected to the ``magic`` function to randomly change the +QLabel's ``text`` property. The `@Slot`` decorator marks +the methods that are slots and informs the QtMetaObject about +them. + +Porting a Qt C++ example +========================= + +Qt offers several C++ examples to showcase its features and help +beginners learn. You can try porting one of these C++ examples to +Python. The +`books SQL example <https://code.qt.io/cgit/qt/qtbase.git/tree/examples/sql/books>`_ +is a good starting point as it does not require you to write UI-specific code in +Python, but can use its ``.ui`` file instead. + +The following chapters guides you through the porting process: + +.. toctree:: + :glob: + :titlesonly: + + chapter1/chapter1 + chapter2/chapter2 + chapter3/chapter3 |