`fontShell` font.generate() raises NotImplementedError, consider adding it

[CorentinBrule]

Environement : fontParts==0.14.1 installed from pip, python 3.13, ubuntu 25.10

Hello ! i'm trying to adapt a RobotFont Extension to work as a standalone python script. Everything seems to be working but i cannot generate fonts. Even if i simplify the script to just open .ufo and try to generate a font.

from fontParts.world import OpenFont

font = OpenFont("Relief-SingleLine.ufo")

font.generate("otfcff","test.otf")

that gives me :

 File "\\\\\\\\\\\\\\\\\\\\\/script.py", line 64, in <module>
    font.generate("otfcff", "test.otf")
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "\\\\\\\\\\\\\\\\\\\\\/.venv/lib/python3.13/site-packages/fontParts/base/font.py", line 367, in generate
    return self._generate(
           ~~~~~~~~~~~~~~^
        format=format,
        ^^^^^^^^^^^^^^
        path=path,
        ^^^^^^^^^^
        environmentOptions=environmentOptions
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "\\\\\\\\\\\\\\\\\\\\\/.venv/lib/python3.13/site-packages/fontParts/base/font.py", line 405, in _generate
    self.raiseNotImplementedError()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "\\\\\\\\\\\\\\\\\\\\\/.venv/lib/python3.13/site-packages/fontParts/base/base.py", line 229, in raiseNotImplementedError
    raise NotImplementedError(
    ...<2 lines>...
    )
NotImplementedError: The RFont subclass does not implement this method.

But generate seems to be.. implemented or i missed a big part of the doc.

[justvanrossum]

I would suggest to do it with fontmake unsteady.

[BoldMonday]

Behind the scenes RoboFont uses makeotf for generating binaries. This application from the Adobe AFDKO can compile UFO files into OTF fonts. And – as mentioned above – Google’s fontmake offers the same functionality. Both of these tools can be installed as stand-alone packages and run from a Python script.

[benkiel]

Hey @CorentinBrule, what you’ve hit is the difference between fontParts being an API that can be adopted by editors, and fontShell which is the command line implementation of fontParts that ships with fontParts. It uses the defcon library to interact with the UFOs, which doesn't have a generate font function. As @justvanrossum mentions, both the FDK and fontmaker will do this, and we should consider adding one of those to fontShell to generate fonts.

Anyone want to pick that up?

I'm changing the issue name to better reflect that.

[justvanrossum]

I would not implement it, as the user is better off with using their own choice of external tool, be it AFDKO, fontmake or fontc. To me, this functionality makes sense inside of RoboFont (where it has a menu equivalent), but far less to outside of it.

[CorentinBrule]

As soon as I have more time, I will get back to you with more details because, in my particular case (single line fonts for CNC drawing) I have been informed that other projects (like SingleLine_otf-svgMaker ) have had difficulties with Google's fontmake and that even with the work of their teams, they seemed unsolvable. It therefore seems that the possibility of choosing a tool to generate font is a good idea.