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Thank you for the question. This is something we briefly considered changing when refactoring things from Gazebo classic to Ignition Gazebo, and decided to stick with the existing XML-based SDFormat to reduce the number of moving pieces.

More generally, I think "the XML question" can be split into 2 broader questions:

  1. changing to a different markup / declarative format like json or yaml
  2. changing to a programming language like python

Your question is related to number 2, so I'm not going to address number 1 now.

While a programmatic way of describing a world could allow users more flexibility, a declarative approach is usually easier to debug and maintain.

One parallel example that's interesting to think about are ROS 2 launch files. ROS 1 used XML, but users wanted more flexibility, so the initial implementation on ROS 2 was python-based. And interestingly, that introduced so much friction that an XML interface was added shortly after. I think their rationale for XML support fits our situation as well:

As an alternative to a programmatic approach to the ROS 2 launch system’s API, a declarative description features a WYSIWYG approach, easier to read, audit and maintain. This is the preferred approach for ROS 1 roslaunch launch files, thus some degree of familiarity is expected (and relied upon).

In any case, I think it would be valuable to eventually support a programmatic way of describing SDF worlds. We currently have a C++ interface to the SDF DOM, so that could be wrapped for a scripting language like python.

As an intermediate solution, we've also been extensively using ERB to template SDF files, which maintains some of the declarative approach while also bringing the power of a fully-fledged programming language (ruby).