In my preprocessing pipeline, I'm setting some information about the subjects. One of the interesting features is the age, or the birth year. But MNE only supports the birthday as a tuple (year, month, day), as per the documentation of mne.Info.

Sadly, I do not have the exact birthday, and instead of setting it as (year, month, 1), I figured I could set a different birthyear or age argument directly.

raw = mne.io.read_raw_fif(...)
raw.info['subject_info']['birthyear'] = 1995
{'id': 65, 'his_id': '065', 'birthyear': 1995, 'sex': 1}
raw.save(...)

And it worked really nicely. Until I saved and reloaded the file.

raw = mne.io.read_raw_fif(...)
raw.info['subject_info']
{'id': 65, 'his_id': '065', 'sex': 1}

The issue is that this emits no warning, no error, and yet the added data is not saved to disk.

If the dictionary stored in 'subject_info' is not used by anything, and is just here for user-defined variables (is this the case?) then how about dropping the limitation on what the keys can be?