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How to make signals appear in QtDesigner?

The PyQt5 docs says
"New signals defined in this way will be automatically added to the
class’s QMetaObject. This means that they will appear in Qt Designer"
How do I make anything appear in QtDesigner if it only knows how to open *.ui files, but not python files?
In my workflow I create ui-file with QtDesigner, then convert it to python using pyuic5.
It is thus a one-way conversion (I then subclass it in another file to avoid my code being overwritten).
How do I make QtDesigner "see" my python code?

Qt Designer recognizes 2 types of elements:
The .ui that are the product,
And the plugins that are the ingredients.
So if you want a widget to be seen in Qt Designer you must create a plugin, if you download the source code in the folder examples/designer/plugins there is an example.
On the other hand there is no converter from .py to .ui since the transformation is not possible in all cases.

Getting events while using Qt Designer

I am developing an application which uses treeview. Initially, I used Qt designer for the GUI and after, compiled the ui file into python. Until now everything was just fine. Now I want to copy and paste the file on pressing Copy and Paste buttons simultaneously.
I watched and read many tutorials but they used re implementation of QPushButton. I can't adopt this technique because I used designer to create GUI and cant make changes in that file. Please help me to figure out that how can i use QEvents of my treeview.
I hope my question is clear.

How to create a file browser in wxpython

I am developing the GUI for my application using wxpython and have most of the features down, except in the main frame/window I want to have a box for choosing a file (in this case, the input will have to be an excel file). Something similar to the standard filebrowser that is accessed whenever you choose "open" from a menu.
Below is an image to show exactly what I want...

You probably want a wx.FileDialog. It provides access to the default file dialog of the OS your app is running in. You can see an example of how it's used in the wxPython demo package. This tutorial also has some screenshots and sample code:
http://www.blog.pythonlibrary.org/2010/06/26/the-dialogs-of-wxpython-part-1-of-2/
The screenshot you show appears to be an interface to actually open the dialog. You can easily create that using sizers and basic widgets. Then just bind the open button to a handler that will show the dialog.
You might also want to take a look at the FileBrowseButton from wx.lib.filebrowsebutton (also in the demo).
There are a few other related widgets which you might be interested in too: wx.DirDialog, MultiDirDialog or wx.GenericDirDialog.

Assuming you know the basics of wxPython you can use wx.GenericDirCtrl and wx.ListCtrl to make nice browser

Creating a multi-screen application using Qt Designer

I'm using Qt Designer to create UI designs which I'm then converting into python code. Since I'm quite new to Qt I'd like to ask: is there a way I could implement a multi-screen application? I.e. having a next button clicked and getting a new set of options/widgets etc within the same window.
To be honest, I was developing using Kivy, and as slick as that is (especially with multiple screens) it depends on PyGame, which proves to be an enormous portability headache, so I had to switch to something else, and PyQt was the next feasible option (or so it seems).

It's called a QWizard. It is not called a multi screen application, but if you search for wizard instead, you find lots of information.
Links
Example with C++ code
[PyQt QWizard documentation] http://pyqt.sourceforge.net/Docs/PyQt4/qwizard.html

IBoutlet with PyObjC and Interface Builder

I'm writing a simple OSX app using Python and PyObjC. I designed the settings dialog using Interface Builder and I use ibtool to compile it, then load it from Python. The problem is how to access the controls I have in this window from the Python code? I played around with iPhone development a bit before and I remember I need to have an IBOutlet in the controller class which will be connected to the UI control in the interface builder. It should look something like this in Python:
class MyClass(NSObject):
my_outlet = objc.IBOutlet('my_outlet')
But since I'm not working in XCode (all I have is a .py file and a .xib file), Interface Builder doesn't know about my outlets. How can I do the binding in this case? Or how else can I access the UI elements from the code?

First, the use of Xcode or not has nothing to do with NIB loading (beyond making it more convenient).
As Ole said, you can use IB to manually add the outlet's you need to file's owner or to the custom object instances that you have in the NIB file. By doing so, it will all "just work".
However, this statement is what prompted my relatively similar answer:
all I have is a .py file and a .xib
file
Are you trying to write a bit of UI code outside of a .app wrapper? If so, that is a wholly unsupported pattern, very difficult to get correct, and quite likely to break across software updates or major releases (as it has many times in the past).
The best way to solve your problem is to use an Xcode project and build a standard application. The templates are no longer shipped with the dev tools. Just download them separately.
If you need to run it from the command line, you can still do so.

I haven't tried this, but you can also define outlets directly in IB. Open the Library panel, select Classes in the segmented control at the top and select your custom class you want to define an outlet for. Let's say you have a NSWindow subclass called MyWindow. Select the NSWindow class in the list, click on the action button at the bottom left, select New Subclass... and name it MyWindow. Now switch to the Outlets tab and create a NSButton outlet for your window. Now you connect a button to the outlet.
I don't know how this will transfer to PyObjC but I'd love to see your results when you try it out.