python: update docs related to using ui files

This changes the tutorial described in the Creator docs
for the UI-based Python application.

The main reason is that loading an ui file is not recommended,
and discouraged over using uic tools to generate Python code
from ui files.

Some changes to code styling were included to the file as well,
mainly the exec_() to exec() changes.

Change-Id: Ibe1f4d62d2252edecd5f3475250f1287d8213faa
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

doc/qtcreator/src/python/creator-python-project.qdocinc

@@ -44,7 +44,7 @@
     use \c {.pyqtc} files, but we recommend that you choose \c{.pyproject} files
     for new projects.
 
-    The \uicontrol {Qt for Python - Window (UI file)} wizard enables you to
+    The \uicontrol {Window UI} wizard enables you to
     create a Python project that contains the source file for a class. Specify
     the PySide version, class name, base class, and and source file for the
     class.
@@ -56,13 +56,23 @@
     Widgets module, and Qt UI tools:
 
     \badcode
-    import os
-    from pathlib import Path
     import sys
+    from pathlib import Path
 
     from PySide6.QtWidgets import QApplication, QWidget
-    from PySide6.QtCore import QFile
-    from PySide6.QtUiTools import QUiLoader
+    \endcode
+
+    \note It is important that you first create the Python code
+    from your UI form. In PySide6, you can do this by executing
+    \c{pyside6-uic form.ui -o ui_form.py} on a terminal. This
+    enables you to import the class that represents your UI
+    from that Python file.
+
+    Once you generate the Python code from the UI file,
+    you can import the class:
+
+    \badcode
+    from ui_form import Ui_Widget
     \endcode
 
     The wizard also adds a main class with the specified name that
@@ -70,25 +80,22 @@
 
     \badcode
     class Widget(QWidget):
-        def __init__(self):
-            super(Widget, self).__init__()
-            self.load_ui()
-            ...
+        def __init__(self, parent=None):
+            super().__init__(parent)
     \endcode
 
-    The following lines in the main class load the generated Python class from
-    the UI file:
+    The following lines in the main class instantiate the generated Python class from
+    your UI file, and set up the interface for the current class.
 
     \badcode
-        def load_ui(self):
-            loader = QUiLoader()
-            path = os.fspath(Path(__file__).resolve().parent / "form.ui")
-            ui_file = QFile(path)
-            ui_file.open(QFile.ReadOnly)
-            loader.load(ui_file, self)
-            ui_file.close()
+            self.ui = Ui_Widget()
+            self.ui.setupUi(self)
     \endcode
 
+    \note UI elements of the new class can be accessed as member variables.
+    For example, if you have a button called \e{button1}, you
+    can interact with it using \c{self.ui.button1}.
+
     Next, the wizard adds a main function, where it creates a
     QApplication instance. As Qt can receive arguments from the command line,
     you can pass any arguments to the QApplication object. Usually, you do not
@@ -96,7 +103,7 @@
 
     \badcode
     if __name__ == "__main__":
-        app = QApplication([])
+        app = QApplication(sys.argv)
     \endcode
 
     Next, the wizard instantiates the \c MainWindow class and shows it:
@@ -107,11 +114,11 @@
         ...
     \endcode
 
-    Finally, the wizard calls the \c app.exec_() method to enter the Qt
+    Finally, the wizard calls the \c app.exec() method to enter the Qt
     main loop and start executing the Qt code:
 
     \badcode
-        sys.exit(app.exec_())
+        sys.exit(app.exec())
     \endcode
 
     You can now modify the boilerplate code in the Edit mode to develop your
@@ -121,6 +128,8 @@
     select \uicontrol {REPL Import File}. To also import all functions from
     the file, select \uicontrol {REPL Import *}.
 
+    Always regenerate the Python code after modifying a UI file.
+
     Open the .ui file in the \uicontrol Design mode to create a widget-based UI
     in \QD.
 
@@ -182,13 +191,13 @@
 
     Finally, the wizard adds code that checks whether the file was successfully
     loaded. If loading the file fails, the application exits with an error code.
-    If loading succeeds, the wizard calls the \c app.exec_() method to enter the
+    If loading succeeds, the wizard calls the \c app.exec() method to enter the
     Qt main loop and start executing the Qt code:
 
     \badcode
     if not engine.rootObjects():
         sys.exit(-1)
-    sys.exit(app.exec_())
+    sys.exit(app.exec())
     \endcode
 
     Open the .qml file in the \uicontrol Edit mode to design a Qt Quick UI, or