fman build system


Technical details on how fbs is meant to be used.

fbs is a Python-based build tool for desktop applications that use PyQt or Qt for Python. It takes your source code and turns it into a standalone executable on Windows, Mac or Linux. It also lets you create an installer for your app on these platforms.

The best place to get started with fbs is the 15 minute tutorial. If you haven't already taken it, it is highly recommended you do so. This Manual is meant as the next, more detailed resource.


fbs runs on Windows, macOS and Linux (Ubuntu, Arch or Fedora). You also need Python 3.5 or 3.6. Python 3.7 is not yet supported.


The easiest (and non-intrusive) way of installing fbs is via pip and a Python virtual environment. To create a virtual environment in the current directory, execute the following command:

python3 -m venv venv

Then, activate the environment with one of the commands below:

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

Next, use pip to install fbs and its dependencies. It is recommended that you use the versions shown in the snippet below. Other versions are untested and would likely cause problems.

pip install fbs PyInstaller==3.4 PyQt5==5.9.2

Built-in commands

Your main point of contact with fbs will likely be its command line. For instance, the command

fbs freeze

turns your application into a standalone executable. Other available commands are startproject, run, clean and installer. Run fbs (without further arguments) to see their descriptions. The tutorial shows them in action.

Start a project

The easiest way to start an fbs project is via the following command:

fbs startproject

This prompts you for a few values:

Commands for starting a new project with fbs

Once you have entered them, fbs creates a skeleton project in the src/ folder of the current directory.

Directory structure

fbs projects use the following directory structure. Parentheses (...) indicate that a file is optional.

  • Root directory for your source files
    • Files for the build process
      • Build settings:
        • all platforms
        • specific to Mac
        • ...
        • all Linux distributions
        • specific to Arch Linux
        • ...
        • ...
    • Implementation of your app
      • Your app's icon.
      • Python code for your application
      • Data files, images etc. See below.
      • Installer source files
      • Files for freezing your app

As you use fbs, you will see that it generates output in a folder called target/, next to src/. Other typical (but not required) files on this directory level can be requirements.txt and venv/.

Using an IDE

The command fbs run is great to quickly run your app. Many people however prefer working in an IDE such as PyCharm. It especially simplifies debugging.

To run an fbs app from other environments (such as an IDE, or the command line), you simply

  • need the virtual environment to be active,
  • have src/main/python on your PYTHONPATH and
  • run src/main/python/

So for example on Mac and Linux, you can also run your app from the command line via

PYTHONPATH=src/main/python python src/main/python/

(assuming the virtual environment is active).

Here are screenshots of how PyCharm can be configured for this:

Your Python code

In order for fbs to find it, your Python source code must lie in src/main/python/. There, you need a script that starts your app with an ApplicationContext. The default script that is generated at src/main/python/ when you run fbs startproject looks as follows:

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

class AppContext(ApplicationContext):           # 1. Subclass ApplicationContext
    def run(self):                              # 2. Implement run()
        window = QMainWindow()
        window.resize(250, 150)
        return                 # 3. End run() with this line

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

The steps 1. – 5. are the minimum of what's required for fbs to work.

As your application grows in complexity, you will likely want to split its source code across multiple files. In this case, it is recommend that you place them all inside one package. For instance, if your app is called My App then the package could be called my_app and the directory structure could look as follows:

  • src/main/python/
    • my_app/
      • ...

If is again the script that instantiates the application context, then you need to change the main_module setting in base.json to "src/main/python/my_app/". This lets fbs know about the new location.

A final tip for more complicated applications: Check out @cached_property. Together with ApplicationContext, it can really help you wire up the components of your app.


Most applications will use extra libraries to implement their functionality. In the typical case, this is as simple as pip install library on the command line and import library in your Python code. When fbs sees the import statement in your code, it automatically packages the dependency alongside your application.

Sometimes, it can happen that automatic packaging does not work for a particular library. That is, fbs run works but running the output of fbs freeze fails. A common symptom of this on Windows is the following dialog when you try to run your frozen app:

'Failed to execute script main' dialog when running a PyQt / PySide app

To debug this, add the --debug flag to freeze:

fbs freeze --debug

When you then start your frozen app from the command line, you should get some debug output. If you see an ImportError mentioning a module xyz, add the following to src/build/settings/base.json:

    ... ,
    "hidden_imports": ["xyz"]

If this does not help to fix a dependency problem, please file an issue. We'll do our best to help.

Resource files

Applications often require data files beyond their source code. Typical examples of this are images displayed in your application. Others are .qss files, Qt's equivalent of CSS.

fbs makes it very easy for you to include data files. Simply place them in one of the following subfolders of src/main/resources/:

  • base/ for files required on all OSs
  • windows/ for files only required on Windows
  • mac/ ...
  • linux/ ...
  • arch/ for files only required on Arch Linux
  • fedora/ ...
  • ubuntu/ ...

When you call fbs freeze, fbs automatically copies the applicable files into your app's frozen directory inside the target/ folder.

To access a resource file from your Python code, simply call ApplicationContext#get_resource(...). The tutorial gives an example of this.

Extra files

In addition to the above, there are other directories you can use to include extra files.

Files in src/freeze/mac/, ... are only included in the frozen version of your app. (So they're not available when you do fbs run.) Their canonical example is Info.plist, a meta file that tells macOS about the name of your application, its version etc.

The folders src/installer/windows/, ... contain files for your installer on the various platforms. For example: The file src/installer/windows/Installer.nsi contains the implementation of the Windows installer.


Consider Info.plist mentioned above. It contains the following lines:


Where does ${app_name} come from? The answer is resource filtering: As fbs copies Info.plist from src/ to target/..., it replaces ${...} by the corresponding setting. For the tutorial, app_name is defined in base.json as follows:

    "app_name": "Tutorial",

So, the Info.plist that ends up in target/ contains Tutorial and not ${app_name}.

To prevent unwanted replacements (eg. in image files), resource filtering is only applied to files listed in the setting files_to_filter. See the file mac.json for an example.

A limitation of resource filtering is that it is not applied during fbs run. In this case, the files you obtain from ApplicationContext#get_resource(...) contain the placeholders unchanged.

Custom commands

At some point, you may want to define your own commands in addition to the built-in ones run, freeze etc. For example, you may want to create a command that automatically uploads the produced binaries to your web site.

fbs lets you define custom commands via the @command decorator. Create a file next to your src/ directory, with the following contents:

from fbs.cmdline import command
from os.path import dirname

import fbs.cmdline

def hi():
    print('Hello World!')

if __name__ == '__main__':
    project_dir = dirname(__file__)

Then, you can execute the following on the command line:

python hi

But also, you can execute all of fbs's built-in commands. For instance:

python run

As your build script grows more complex, it is recommended that you split it into two parts: Put the command definitions (the "what") into and their implementation (the "how") into src/build/python. (If you use this approach, you will also have to add src/build/python to sys.path at the beginning of


fbs consists of two Python packages: fbs and fbs_runtime. The first implements the built-in commands. The second contains logic for actually running your app on your users' computers.

When you use fbs, you typically add references to fbs_runtime to your source code. For example, the default does this with ApplicationContext from this package.

On the other hand, your code does not necessarily have to mention the fbs package. Some functions in this package however are exposed to let you define custom commands, or to modify the behaviour of fbs's built-in ones.

What you usually don't want to do is to refer to fbs from your application's implementation (src/main/). If at all, you should only refer to fbs from build scripts ( and/or src/build/python/).


This class is the main point of contact between fbs and your application. As mentioned above, fbs requires you to subclass, instantiate and run it. It lies in the module fbs_runtime.application_context and has the following methods and properties:

You must implement this method to use fbs. In it, execute all steps that are required for starting your app. Typically, this involves creating a window and calling .show() on it. Your implementation must end with return A minimal implementation can be found above.


This method returns the absolute path to the resource file with the given name or (relative) path. For example, if you have src/main/resources/base/image.jpg and call get_resource('image.jpg'), then this method returns the absolute path to the image. If the given file does not exist, a FileNotFoundError is raised.

The implementation of this method transparently handles the different subdirectories of src/main/resources. That is, if image.jpg exists in both src/main/resources/base and src/main/resources/mac and you call it on Mac, you obtain the absolute path to the latter.

This method also works both when you run your app from source (via fbs run), or when your users run the compiled form of your app. In the first case, the path in src/main/resources is returned. In the second, the path to the given file in your app's installation directory. Do note that the files are only filtered in the latter case.

This property holds the global QApplication object for your app. Every Qt GUI application has precisely one such object. fbs ensures that it is automatically instantiated.

You can use this property to access the QApplication object. The canonical example of this is when you call at the end of your implementation of run().

Another reason why this property is exposed by fbs is that this lets you overwrite it. For instance, you may want to use a custom subclass of QApplication. Here is how you might integrate it:

from fbs_runtime.application_context import ApplicationContext, cached_property
from PyQt5.QtWidgets import QApplication

class AppContext(ApplicationContext):
    def app(self):
        return MyCustomApp(sys.argv)

class MyCustomApp(QApplication):

For more information about @cached_property, see below.


Every application developer needs to answer the following question: How do I wire up the different objects that make up my application? fbs's answer to this is an interplay of the ApplicationContext class and @cached_property.

fbs encourages (but does not force) you to instantiate all components in your application context. For example: Say you have a Window class, which displays some information from a Database. Your application context could look as follows:

from fbs_runtime.application_context import ApplicationContext, cached_property

class AppContext(ApplicationContext):
    def window(self):
        return Window(self.db)
    def db(self):
        return Database()
    def run(self):

When run() is invoked when your application starts, its first line accesses self.window. This executes the code in the definition of the window(...) property. This accesses self.db, which in turn executes the code in the definition of db(...). The end result is that we instantiate both Database and Window, without a long stream of spaghetti code.

Taking a step back, we see that the application context becomes the "home" for all of your application's components. Making it the one (and only) place where components are instantiated makes it extremely easy to see how the different parts of your application are connected. What's more, @cached_property ensures that each component is only created once: Subsequent calls to the same property return the result of the previous access.

Technically speaking, @cached_property is simply a Python @property that caches its results.

Having a central place / mechanism for wiring up components is a well-known technique called Dependency Injection. For further information, see for instance this article.

Module fbs_runtime.platform

This module exposes several functions that let you determine the platform your app is executing on. Their names should be pretty self-explanatory:

  • is_windows()
  • is_mac()
  • is_linux()
  • is_ubuntu()
  • is_arch_linux()
  • is_fedora()
  • is_gnome_based()
  • is_kde_based()

Another function of potential interest is name(). It returns 'Windows', 'Mac' or 'Linux', depending on the current operating system.


The most important parts of fbs's API are described above. But there are more functions which you can use. They are documented in (extensive) comments in fbs's source code. You can consider everything whose name doesn't start with an underscore _ a part of the public API.