class BIDSFile(object):
 # as defined in https://bids-specification.readthedocs.io/en/stable/99-appendices/04-entity-table.html
 # which might soon become machine readable
 # order matters
 _known_entities = ['sub', 'ses', ...]
 def __init__(self, entities, suffix, extension):
   self._entities = entities
   self._suffix = suffix
   self._extension = extension

 @classmethod
 def parse(filename):
     # use re.findall to find all key pairs, and then leftover might start with _ which would be suffix
     return BIDSFile(entities, suffix, extension)
 def __str__(self):
     # here reconstitute in a legit BIDS filename using the order from entity table
     # using self._known_entities
     # puke if mandatory `sub` is not known!
 def __getitem__(self, entity):
     return self._entities[entity]
 def __setitem__(self, entity, value):  # would puke with some exception if already known
     return self.set(entity, value, overwrite=False)
 def set(self, entity, value, overwrite=True):
     ...
 @property  # as needed make them RW
 def suffix(self):
    return self._suffix 
 @property 
 def extension(self):
    return self.extension