ml_collections / absl -- cannot read config updates from --flagfile

[danielkelshaw]

Describe the bug
When using config_flags.DEFINE_config_dict we are unable to use --flagfile to override values, we get the error:

FATAL Flags parsing error: Unknown command line flag ...

To Reproduce

# test.py

from absl import app, flags
from ml_collections import config_dict, config_flags

import copy


CONFIG = config_dict.ConfigDict(
    {
        'animal': {
            'cat': 10,
            'dog': 20
        },
        'vehicle': {
            'car': 30,
            'train': 40
        }
    }
)


FLAGS = flags.FLAGS
# _CONFIG = config_flags.DEFINE_config_dict('config', copy.deepcopy(CONFIG))
_CONFIG = config_flags.DEFINE_config_dict('config', CONFIG)


def main(_):

    print(FLAGS.config)


if __name__ == '__main__':
    app.run(main)

# flagfile.txt
--config.animal.cat=5000

$ python test.py --flagfile=flagfile.txt

FATAL Flags parsing error: Unknown command line flag 'config.animal.cat'
Pass --helpshort or --helpfull to see help on flags.

Expected behavior
absl should be able to read the changes to config from the flagfile.txt, something is going wrong here.

Environment:

• OS: MacOS
• OS Version: macOS Venture (13.0)
• Python: 3.10.0