Tutorial for creating Python/Qt GUIs with fbs
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README.md

fbs tutorial

This tutorial shows how you can use fbs to create a simple Python GUI and an associated installer:

Screenshot of sample app on Windows Windows installer

You can follow this tutorial on Windows, Mac or Linux. The only prerequisite is Python, version 3.5. Later versions have known issues. Use them at your own risk.

Setup

Create a virtual environment in the current directory:

python3 -m venv venv

Activate the virtual environment:

# On Mac/Linux:
source venv/bin/activate
# On Windows:
call venv\scripts\activate.bat

The remainder of the tutorial assumes that the virtual environment is active.

Install the required libraries (most notably, fbs and PyQt5):

pip install fbs PyQt5==5.9.2 PyInstaller==3.4

You can also use Qt for Python instead of PyQt. To do this, simply write PySide2 instead of PyQt5 throughout this tutorial. For the above, use pip install PySide2==5.11.2.

Start a project

Execute the following command to start a new fbs project:

python -m fbs startproject

This asks you a few questions. You can for instance use Tutorial as the app name and your name as the author.

The command creates a new folder called src/ in your current directory. This folder contains the minimum configuration for a bare-bones PyQt app.

Run the app

To run the basic PyQt application from source, execute the following command:

python -m fbs run

This shows a (admittedly not very exciting) window. Screenshots on Windows/Mac/Ubuntu:

Screenshot of sample app on Windows Screenshot of sample app on Mac Screenshot of sample app on Ubuntu

Source code of the sample app

Let's now take a look at the source code of the PyQt app that was generated. It is at src/main/python/main.py:

from fbs_runtime.application_context import ApplicationContext
from PyQt5.QtWidgets import QMainWindow

import sys

class AppContext(ApplicationContext):           # 1. Subclass ApplicationContext
    def run(self):                              # 2. Implement run()
        window = QMainWindow()
        window.setWindowTitle('Hello World!')
        window.resize(250, 150)
        window.show()
        return self.app.exec_()                 # 3. End run() with this line

if __name__ == '__main__':
    appctxt = AppContext()                      # 4. Instantiate the subclass
    exit_code = appctxt.run()                   # 5. Invoke run()
    sys.exit(exit_code)

The important steps are highlighted as comments. If they look daunting to you, don't worry. They're the only boilerplate that's required. In the middle of the code, you can see that a window is being created, resized and then shown.

Freezing the app

We want to turn the source code of our app into a standalone executable that can be run on your users' computers. In the context of Python applications, this process is called "freezing".

Use the following command to turn the app's source code into a standalone executable:

python -m fbs freeze

This creates the folder target/Tutorial. You can copy this directory to any other computer (with the same OS as yours) and run the app there! Isn't that awesome?

Creating an installer

Desktop applications are normally distributed by means of an installer. On Windows, this would be an executable called TutorialSetup.exe. On Mac, mountable disk images such as Tutorial.dmg are commonly used. Finally on Ubuntu, .deb files are the de-facto standard. fbs lets you generate each of these files via the command:

python -m fbs installer

Windows installer

Before you can use the installer command on Windows, please install NSIS and add its installation directory to your PATH environment variable.

The installer is created at target/TutorialSetup.exe. It lets your users pick the installation directory and adds your app to the Start Menu. It also creates an entry in Windows' list of installed programs. Your users can use this to uninstall your app. The following screenshots show these steps in action:

Mac installer

On Mac, the installer command generates the file target/Tutorial.dmg. When your users open it, they see the following volume:

Screenshot of installer on Mac

To install your app, your users simply drag its icon to the Applications folder (also shown in the volume).

Linux installer

On Linux, the installer command requires that you have fpm. You can for instance follow these instructions to install it.

Depending on your Linux distribution, fbs creates the installer at target/Tutorial.deb, ...pkg.tar.xz or ...rpm. Your users can use these files to install your app with their respective package manager.

A more interesting example

We will now create a more powerful example. Here's what it looks like on Windows:

Quote app

When you click on the button in the window, a new quote is fetched from the internet and displayed above.

Before you can run this example, you need to install the Python requests library. To do this, type in the following command:

pip install requests

The source code of the new app consists of two files:

Please copy the former over the existing file in src/main/python/, and the latter into the new directory src/main/resources/base/. If you are using PySide2 instead of PyQt, you have to replace all occurrences of PyQt5 in main.py by PySide2.

Once you have followed these steps, you can do python -m fbs run (or ... freeze etc.) as before.

The new app uses the following code to fetch quotes from the internet:

def _get_quote():
    response = requests.get('https://talaikis.com/api/quotes/random/')
    return response.json()['quote']

You can see that it uses the requests library we just installed above. Feel free to open talaikis.com/api/quotes/random in the browser to get a feel for its data.

The app follows the same basic steps as before. It defines an application context with a run() method that ends in return self.app.exec_():

class AppContext(ApplicationContext):
    def run(self):
        ...
        return self.app.exec_()
    ...

It then instantiates this application context and invokes run():

if __name__ == '__main__':
    appctxt = AppContext()
    exit_code = appctxt.run()
    sys.exit(exit_code)

What's different is what happens in between. First, let's look at the implementation of run():

def run(self):
    stylesheet = self.get_resource('styles.qss')
    self.app.setStyleSheet(open(stylesheet).read())
    self.window.show()
    return self.app.exec_()

The first line uses get_resource(...) to obtain the path to styles.qss. This is a QSS file, Qt's equivalent to CSS. The next line reads its contents and sets them as the stylesheet of self.app.

fbs ensures that get_resource(...) works both when running from source (i.e. during python -m fbs run) and when running the compiled form of your app. In the former case, the returned path is in src/main/resources. In the latter, it will be in your app's installation directory. fbs handles the corresponding details transparently.

The last but one line accesses self.window. This is defined as follows:

@cached_property
def window(self):
    return MainWindow()

You can use @cached_property to define the components that make up your app. The way it works is that the first time self.window is accessed, return MainWindow() is executed. Further accesses then cache the value and return it without re-executing the code.

This approach is extremely powerful: In your ApplicationContext, define a @cached_property for each component (a window, a database connection, etc.). If it requires other objects, access them as properties. For example, if the window requires the database because it displays information from it, then its @cached_property would access self.database. If you connect the parts of your application in this centralised way, then it is extremely easy to see how they work together.

The final bit of code is the definition of MainWindow. It sets up the text field for the quote and the button. When the button is clicked, it changes the contents of the text field using _get_quote() above. You can find the full code in main.py.

As already mentioned, you can use python -m fbs run to run the new app. But here's what's really cool: You can also do python -m fbs freeze and ... installer to distribute it to other computers. fbs includes the requests dependency and the styles.qss file automatically.

Summary

fbs lets you use Python and Qt to create desktop applications for Windows, Mac and Linux. It can create installers for your app, and automatically handles the packaging of third-party libraries and data files. These things normally take weeks to figure out. fbs gives them to you in minutes instead.

Where to go from here

fbs's Manual explains the technical foundation of the steps in this tutorial. Read it to find out more about fbs's required directory structure, dependency management, handling of data files, custom build commands, API and more.

If you have not used PyQt before: It's the library that allowed us in the above examples to use Qt (a GUI framework) from Python. fbs's contribution is not to combine Python and Qt. It's to make it very easy to package and deploy PyQt-based apps to your users' computers. For an introduction to PyQt, see here.

Feel free to share the link to this tutorial! If you are not yet on fbs's mailing list and want to be notified as it evolves, sign up here.