Develop Reference

Add side panel without resizing parent

I'm using PySide2 to write a gui (witout QtDesigner).
The mainwindow contains plots and some other widgets. Through a menu option I want to open a side panel widget. The way I want it to work is that the whole window simply grows to contain that new widget without changing the size of anything else in the main window. How can this be done?
Currently the widget is just added to the central layout with addWidget, I've also tried making it a QDockWidget but it is still resized (and anyway I would like to avoid the extra fluff that comes with having a DockWidget).
So I have
---------
|content|
---------
which should turn into
-------------
|new|content|
-------------
but currently I get
---------
|new|cnt|
---------

It's hard to do well on the "client" side of Qt; this would really belong within Qt itself. I have implemented a slightly more general variant of this a couple of years ago, and just to make it work well across Windows, Mac and KDE, the code ballooned to over a thousand lines to cover all the icky corner cases, with another thousand for the test harness. It was surprisingly hard to implement the tests - especially on X11, where there was no way around using native X APIs to verify intended behavior. I got rid of that monstrosity soon later - the effort was unnecessary.
You can have the side panel as a separate top-level frameless widget that moves itself so that its top-right corner is aligned with the top-left corner of the content window, and resizes itself vertically to match the vertical size of the content window. You can of course make it slightly shorter (vertically) while still center-aligning it vertically with the content window.
You'd want to capture the resize events of the content window to do this: the side panel should install itself as an event filter for the content window.
You'll want the side panel to be a Qt child of the content window, but you also need to make it a top-level window, i.e. set the Qt::Window flag on it, so that it becomes top-level and not a sub-widget of the content window.

Resizing an icon doesn't show on GUI

I am trying to make my GUI icon go bigger.
I tried this:
MainWindow.setWindowIcon(QtGui.QIcon('Logo1.png'))
MainWindow.setIconSize(QtCore.QSize(128,128))
When 'Logo1.png' is 128x128
When I change numbers SetIconSize line, like this:
MainWindow.setIconSize(QtCore.QSize(500,500))
It doesn't show on my GUI.
My questions are:
Does this happen because I need my logo to be smaller something like 28X28?
If I need a specific size, what size is it and how do I make my logo this size?
Even if I do need a specific size, why wont setIconSize change my icon size?

The iconSize property documentation of QMainWindow explains that:
[The] size of toolbar icons in this mainwindow.
As you can see, it has nothing to do with the windowIcon.
It is up to the underlying OS and its window manager to decide the size of the icon, whether its shown in the window decoration (tipically in the title bar), the task manager/window switcher or anything else, and you don't have any control over it through Qt.
The only "exception" is when drawing client-side windows: windows for which the whole decoration is drawn by the program (the title bar with its system buttons and icon, the frame around the window, etc.).
That is, though, something that is usually discouraged as it's hard to achieve without facing various difficulties (both with drawing and interaction); it also makes the window appearance inconsistent with the whole system and could also create issues with accessibility for visually impaired people.

How to avoid layouts to change their size?

I set a QMainWindow with two layouts, called A and B, in a Pyqt4 app written in Python 3.5
They usually display as shown in this schema:
But at some point, something in Layout B makes wider. Therefore Layout B makes wider too, overlapping Layout A, which remain partially hidden:
Could anybody please give me a hand to find a solution to this behaviour?
I would like to lock Layout A and B widths somehow to avoid this issue.

Without seeing your code, I suspect you add both layouts directly to your main window, without putting them into a main layout that would control their resizing. This way you're creating unnecessary problems, as you need to manage the layout's position and size manually. Read more in the Qt docs.
To solve this, you just add a QHBoxLayout to the main window, then the child layouts to that using addLayout() with an appropriate stretch factor. Here's an example in C++ (untested) to illustrate the necessary steps:
QHBoxLayout* mainLayout = new QHBoxLayout();
QGridLayout* childLayout0 = new QGridLayout();
QGridLayout* childLayout1 = new QGridLayout();
//Add widgets to the child layouts
mainLayout->addLayout(childLayout0, 2);
mainLayout->addLayout(childLayout1, 1);
mainWindow->setLayout(mainLayout);

Implementing Gedit style fullscreen HeaderBar

I am working on a Gtk+ 3 application that is likely to be used fullscreen most of the time, but needs to be switched between fullscreen and non-fullscreen, while maintaining access to the controls located in the header bar. The problem is, since the headerbar is part of the window decorations, it gets hidden when the window goes fullscreen.
My current kludge so ensure the controls are always available works like this:
Setup
create a Gtk.Window with vertical Gtk.Box as first child
create a custom Gtk.HeaderBar (w/ added full screen togglebutton)
set window's titlebar as my custom Gtk.HeaderBar
add all the window's content to the Gtk.Box
When window goes fullscreen
remove the Gtk.HeaderBar from the Gtk.Window titlebar
pack the Gtk.HeaderBar into the Gtk.Box (window's first child).
This results in the Gtk.HeaderBar being at the bottom of the window, so
re-position the Gtk.HeaderBar to the top of the Gtk.Box
When the window goes un-fullscreen
remove the Gtk.HeaderBar from the gtk.Box
set it as the Gtk.Window's titlebar
This results in the following Gtk-warning: gtk_window_set_titlebar() called on a realized window (who cares, just a warning)
This works, but it seems like very much of a hack, and more complicated than it should have to be. Am I missing something and there is a more straightforward approach?
I know several Gtk+ 3 based apps have the header bar behavior I am after (gedit for example), but I have not been able to determine how that is implemented. Any insight would be greatly appreciated.
Also, here is a GitHub gist with a full working example of my current hacky solution: https://gist.github.com/KurtJacobson/6b045b6fc38907a2f18c38f6de2929e3
I will accept answers in any (programming) language.

How to have 2 QTreeview widgets side by side and resize

I am using QTDesigner to design an application (I am trying my hand at a dual pane file manager). I can't figure out how to have the 2 widgets side by side so that they both resize when I resize the application

A layout, as JRazor mentioned, is a good solution if you want your Tree Views to have always same size. If not, use a QSplitter
From the Qt documentation:
A splitter lets the user control the size of child widgets by dragging
the boundary between them.
QSplitter *splitter = new QSplitter(parent);
QListView *listview = new QListView;
QTreeView *treeview = new QTreeView;
splitter->addWidget(listview);
splitter->addWidget(treeview);
EDIT
Sorry I didn't notice that you are actually looking for a python solution. I have provided a C++ example code, but I believe that it's not that big difference doing it with python.

Use layout:
layout = QHBoxLayout(self)
layout.addWidget(left_tree)
layout.addWidget(right_tree)
If you want use QtDesigner: http://doc.qt.io/qt-4.8/designer-layouts.html