Update "child" params when "parent" params are modified

[andretag]

Thank you for your great software! I often find myself needing a pattern where I define some "child" parameter that depends on other "parent" parameters; is it possible/is there a way to ensure that a "child" parameter (e.g., B) that depends on other parameters (e.g., A) gets updated when the "parent" parameter (A) is updated?

For example:

# in config.py
from yacs.config import CfgNode as CN
_C  = CN()

_C.A = 1.0
_C.B = 2.0*_C.A
cfg = _C

# in main.py
from config import cfg

if __name__ == "__main__":
  cfg.merge_from_list(["A", 3.0])
  cfg.B # <-------- Expected 6.0, it is 2.0

Thanks,
Andrea