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 PySide. 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.

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. The free version of fbs supports Python 3.5 and 3.6. Later Python versions require fbs Pro.


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:

pip install fbs PyQt6

You can similarly install PySide6, PyQt5 or PySide2. Using PyQt6 or PySide6 requires fbs Pro.

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.

src/ Root directory for your source files
build/ Files for the build process
settings/ Build settings:
base.json all platforms
(mac.json) specific to Mac
(windows.json) ...
(linux.json) all Linux distributions
(arch.json) specific to Arch Linux
(fedora.json) ...
(ubuntu.json) ...
(release.json) during a release
main/ Implementation of your app
icons/ Your app's icon.
python/ Python code for your application
(resources/) Data files, images etc. See below.
(freeze/) Files for freezing your app
(installer/) Installer source files
(requirements/) Your Python dependencies:
(base.txt) on all platforms
(linux.txt) on all Linux distributions
(arch.txt) on Arch Linux
... ...

As you use fbs, you will see that it generates output in a folder called target/, next to the above directories. It may also create a folder called cache/, which you can delete whenever you want. Finally, another typical (but not required) folder on this level is 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.PyQt6 import ApplicationContext
from PyQt6.QtWidgets import QMainWindow

import sys

if __name__ == '__main__':
    appctxt = ApplicationContext()       # 1. Instantiate ApplicationContext
    window = QMainWindow()
    window.resize(250, 150)
    exit_code =       # 2. Invoke

The steps 1. and 2. 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, 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 search online for "<your library> PyInstaller". (fbs uses PyInstaller to package dependencies.) If that doesn't turn anything up, you may be able to get help on PyInstaller's issue tracker.

Declaring dependencies

Once you are using a new library in your project, it is recommended that you add it to the requirements/ folder. For example, say you are on Windows and have added the Windows-only dependency somelibrary. Then you would create the requirements/ folder next to src/, copy base.txt into it and also create the following file windows.txt there:

-r base.txt

The advantage this has is that other developers who check out your repository from version control can then quickly get all required Python libraries via the command:

pip install -r requirements/windows.txt

Further, some fbs commands such as buildvm can only take your dependencies into account if you follow the above structure.

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.

Releasing your appPro

The above descriptions and the tutorial lead up to the creation of an installer. But publishing a production grade app requires several more steps:

  • Modern OSs require you to code sign your application. This avoids ugly warnings "untrusted app" when your users download and run it.
  • Your installer needs to be uploaded to a server, so users can download it.
  • New releases of your app should automatically be rolled out to existing users.

fbs accomplishes all of the above tasks when you run the following command:

fbs release

However, this command requires a few preparations.

The first step is to install a few more dependencies. To do this, please execute:

pip install fbs[upload]

Next, create an fbs account via the command:

fbs register

Alternatively, if you already have an account, you can use fbs login to set its credentials.

The remaining preparations depend on your target operating system. They are discussed separately below.


Automatic updates are not yet implemented on Windows. If you want to code sign your application, please see the relevant section. Otherwise, no special setup is required. Just run fbs release to publish your app.


fbs does not yet implement either code signing or automatic updates on Mac, so no special setup is required. Simply execute fbs release to publish your app.

Unlike Windows, there is one thing to take into account when publishing your app: Try to build it on as old a version of macOS as possible. This improves the compatibility of your app with older versions of macOS. For example, an app built on macOS 10.10 is most likely to also run on 10.14, but not the other way around. Most people use virtual machines to run old versions of macOS.


Unlike on the other platforms, fbs fully implements code signing and automatic updates on Linux. The following gives you a minimal but complete example:

fbs startproject
fbs gengpgkey
fbs register       # or `login` if you already have an account
fbs buildvm ubuntu # or `arch` / `fedora`
fbs runvm ubuntu
# In the Ubuntu virtual machine:
fbs release

After the above, users on Ubuntu can install your app with the following commands:

wget -qO -<user>/<app>/public-key.gpg | sudo apt-key add -
echo 'deb [arch=amd64]<user>/<app>/deb stable main' \
  | sudo tee /etc/apt/sources.list.d/<app>.list
sudo apt-get update
sudo apt-get install <app>

What's more, users who install your app in this way automatically receive updated versions through their native package manager.

You can infer from the gengpgkey command above that fbs uses a GPG key. This is the standard for code signing on Linux.

The next command, register, lets you create an account for fbs's backend. This is required so only you can modify your app.

The buildvm and runvm commands

Apps built on Ubuntu 16 usually run on Ubuntu 18, but not the other way around. The buildvm and runvm commands let you create and start virtual machines running older Linux versions. For example, runvm ubuntu starts a virtual machine running Ubuntu 16. Building your app there maximises its compatibility.

Using virtual machines for releasing your app has another benefit: The VM serves as an isolated environment. This makes your builds more reproducible. Also, it prevents you from having to install tools that are only required during a release. And from having to import GPG keys.

The two VM commands are implemented using Docker. You can see this when you run docker images. For example, after buildvm ubuntu:

michael:~$ docker images
REPOSITORY         TAG          IMAGE ID          CREATED           SIZE
myapp/ubuntu       latest       30abafe515e8      19 hours ago      1.03GB

The command buildvm ubuntu builds a Docker image according to the instructions in src/build/docker/ubuntu/Dockerfile. The default implementation performs the following steps:

  1. Install necessary tools such as Python.
  2. Set up a virtual environment in the venv/ directory.
  3. Install the Python dependencies listed in requirements/.
  4. Import the GPG key of your app for code signing.

When you then do runvm ubuntu, fbs mounts the files from your project directory inside the container and starts it. Because your files are mounted, any outside changes you make to eg. src/... are immediately visible. So eg. freeze always uses the current version of your source code, even when run in a container.

On the other hand, some changes are not immediately visible in the container. For example, the virtual environment is only updated when you call buildvm. So you need to re-run this command after adding Python dependencies. Similarly for when you set or change the GPG key.

Because of the way Docker works, runvm always starts from the state created by buildvm. This means that any changes you make inside runvm are lost as soon as you type exit. So while it may be tempting to call pip install somelibrary inside the container, the results of this command will be short-lived. As mentioned above, you need to add the dependency to requirements/ instead.

The final caveat applies to the folders venv/ and target/ inside the Docker container. Unlike other project files such as src/, these two directories are not just mounted into the container. The target/ folder actually points to target/ubuntu (or .../arch etc.) in your project directory. And venv/ does not exist outside the Docker container at all.

Releasing on multiple platforms

fbs lets (and in fact encourages) you to release your app on multiple operating systems. The caveat is that fbs commands only ever target the current platform. That is, you for instance cannot create a Mac installer when running fbs on Windows. The solution to this is to use virtual machines to invoke fbs on different platforms. A video of fbs's creator shows this in practice.

Code signingPro

Code signing is required to avoid warnings by the user's OS that your app is untrusted:

Windows Defender SmartScreen

For code signing on Linux, see the section on releasing for Linux above. On macOS, fbs does not (yet) implement code signing. For instructions on Windows, see below.

Code signing on Windows

On Windows, code signing certificates usually come in the form of a .pfx file. To use it with fbs, place it at src/sign/windows/certificate.pfx. Then, set "windows_sign_pass" to the password for this file in either src/build/settings/secret.json, .../windows.json or .../base.json. Optionally, you can also set "windows_sign_server" to the timestamp server that should be used for signing. For example: "".

Next you need to ensure you have Windows's signtool and that it is on your PATH. For instructions how to do this, please see here.

Once you have performed these steps, you can use the commands fbs sign and fbs sign_installer to code sign your app's frozen binaries and its installer, respectively.

Error trackingPro

Once your app is installed on somebody else's computer, you will want to know when errors (/exceptions) occur running your app. With associated stack traces, this can be invaluable for learning about problems and fixing them.

fbs can upload errors that occur in your app to Sentry. This gives you a web interface for inspecting exceptions and stack traces.

To enable error tracking for your app, create a Sentry account and project. This gives you a DSN / Client Key similar to:

Save this as a setting to your src/build/settings/base.json. Also add the setting's name to public_settings. For example:

    "sentry_dsn": "",
    "public_settings": ["sentry_dsn"]

Next, install the necessary dependencies via:

pip install fbs[sentry]

(Don't forget to also add this dependency to your requirements/.)

Now you can add SentryExceptionHandler to the exception_handlers property of your ApplicationContext:

from fbs_runtime import PUBLIC_SETTINGS
from fbs_runtime.application_context import cached_property, \
from fbs_runtime.application_context.PyQt6 import ApplicationContext
from fbs_runtime.excepthook.sentry import SentryExceptionHandler

class AppContext(ApplicationContext):
    def exception_handlers(self):
        result = super().exception_handlers
        if is_frozen():
        return result
    def sentry_exception_handler(self):
        return SentryExceptionHandler(

This only sends errors for the frozen (i.e. compiled) form of your app. fbs automatically sets the environment setting to either local for when you're developing locally, or production for the release version of your app. This lets you distinguish the two in Sentry.

Often, you want extra information such as the user's operating system when an exception occurs. You can set this via the .scope property of the Sentry exception handler. It is only available once the exception handler was initialized, so you need to use the callback parameter:

def sentry_exception_handler(self):
    return SentryExceptionHandler(..., callback=self._on_sentry_init)
def _on_sentry_init(self):
    scope = self.sentry_exception_handler.scope
    from fbs_runtime import platform
    scope.user = {'id': 41, 'email': ''}

For more information about the additional data you can log this way, see Sentry's documentation.

License keysPro

Commercial desktop applications often require a license protection scheme. This prevents users who have not yet bought your app from using it. The typical vehicle for this are license keys: When a user purchases your app, you send them a license key that unlocks your application.

fbs makes it very easy for you to implement a reasonably secure license scheme. To do this, first install the necessary Python dependencies:

pip install fbs[licensing]

(Don't forget to also add this dependency to your requirements/.)

Then, use fbs's init_licensing command to generate a public/private key pair. This will be used for creating and verifying your license keys:

fbs init_licensing

The workflow then is as follows:

When a user buys your app, generate a license key via the Python code

# This file was generated by `init_licensing` above:
secret_json = 'src/build/settings/secret.json'
import json
privkey = json.load(open(secret_json))['licensing_privkey']
from fbs_runtime.licensing import pack_license_key
print(pack_license_key({'email': ''}, privkey))

Say the user saves the output of the above print(...) statement at C:\license.key. Then your application can verify that the user is licensed with the following code:

from fbs_runtime import PUBLIC_SETTINGS
from fbs_runtime.licensing import unpack_license_key

def get_license_key():
    with open(r'C:\license.key') as f:
        key_contents =
    return unpack_license_key(key_contents, PUBLIC_SETTINGS['licensing_pubkey'])

This raises FileNotFoundError or fbs_runtime.licensing.InvalidKey if the user does not have a valid license key. Otherwise, it returns the key data {'email': ...}. For background information about fbs's implementation, see this article.

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 instantiate it. It lies in the module fbs_runtime.application_context.PyQt6 (or fbs_runtime.application_context.PySide6 if you are using PySide6) and has the following methods and properties:


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, as required by fbs.

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 cached_property
from fbs_runtime.application_context.PyQt6 import ApplicationContext
from PyQt6.QtWidgets import QApplication

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

class MyCustomApp(QApplication):

if __name__ == '__main__':
    appctxt = MyAppContext()

For more information about @cached_property, see below.


This dict-like object exposes some build settings. A common use case is to display your app's version:

from fbs_runtime import PUBLIC_SETTINGS
print('Starting version ' + PUBLIC_SETTINGS['version'])

The available settings are controlled by the setting "public_settings". Eg., in the default base.json:

"public_settings": ["app_name", "author", "version"]

To extend this list, simply re-define it in one of your own settings files, eg. base.json. You don't have to repeat the elements above because fbs automatically concatenates lists defined in multiple settings files.

The motivation for only making some settings "public" is that settings often contain secret information such as passwords. We don't want these to be included in the frozen form of your app.


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 cached_property
from fbs_runtime.application_context.PyQt6 import ApplicationContext

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

if __name__ == '__main__':
    appctxt = AppContext()

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.