diff --git a/heudiconv/bids/schema.py b/heudiconv/bids/schema.py
index 3c271b98..82864302 100644
--- a/heudiconv/bids/schema.py
+++ b/heudiconv/bids/schema.py
@@ -24,7 +24,150 @@ class BIDSFile:
     _known_entities = _load_entities_order()
 
 
+
+    def __init__(self, entities, suffix, extension):
+        self._entities = entities
+        self._suffix = suffix
+        self._extension = extension
+
+    def __eq__(self, other):
+        if not isinstance(other, self.__class__):
+            return False
+        if (
+            all([other[k] == v for k, v in self._entities.items()])
+            and self.extension == other.extension
+            and self.suffix == other.suffix
+        ):
+            return True
+        else:
+            return False
+
+    @classmethod
+    def parse(cls, filename):
+        """ Parse the filename for BIDS entities, suffix and extension """
+        # use re.findall to find all lower-case-letters + '-' + alphanumeric + '_' pairs:
+        entities_list = re.findall('([a-z]+)-([a-zA-Z0-9]+)[_]*', filename)
+        # keep only those in the _known_entities list:
+        entities = {k: v for k, v in entities_list if k in BIDSFile._known_entities}
+        # get whatever comes after the last key-value pair, and remove any '_' that
+        # might come in front:
+        ending = filename.split('-'.join(entities_list[-1]))[-1]
+        ending = remove_prefix(ending, '_')
+        # the first dot ('.') separates the suffix from the extension:
+        if '.' in ending:
+            suffix, extension = ending.split('.', 1)
+        else:
+            suffix, extension = ending, None
+        return BIDSFile(entities, suffix, extension)
+
+    def __str__(self):
+        """ reconstitute in a legit BIDS filename using the order from entity table """
+        if 'sub' not in self._entities:
+            raise ValueError('The \'sub-\' entity is mandatory')
+        # reconstitute the ending for the filename:
+        suffix = '_' + self.suffix if self.suffix else ''
+        extension = '.' + self.extension if self.extension else ''
+        return '_'.join(
+            ['-'.join([e, self._entities[e]]) for e in self._known_entities if e in self._entities]
+        ) + suffix + extension
+
+    def __getitem__(self, entity):
+        return self._entities[entity] if entity in self._entities else None
+
+    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):
+        if entity not in self._entities:
+            # just set it; no complains here
+            self._entities[entity] = value
+        elif overwrite:
+            lgr.warning("Overwriting the entity %s from %s to %s for file %s",
+                        str(entity),
+                        str(self[entity]),
+                        str(value),
+                        self.__str__()
+                        )
+            self._entities[entity] = value
+        else:
+            # if it already exists, and overwrite is false:
+            lgr.warning("Setting the entity %s to %s for file %s failed",
+                        str(entity),
+                        str(value),
+                        self.__str__()
+                        )
+
+    @property  # as needed make them RW
+    def suffix(self):
+        return self._suffix
+
+    @property
+    def extension(self):
+        return self._extension
+
 # TEMP: just for now, could be moved/removed
 def test_BIDSFile():
+    """ Tests for the BIDSFile class """
+
+    # define entities in the correct order:
+    sorted_entities = [
+        ('sub', 'Jason'),
+        ('acq', 'Treadstone'),
+        ('run', '2'),
+        ('echo', '1'),
+    ]
+    # 'sub-Jason_acq-Treadstone_run-2_echo-1':
+    expected_sorted_str = '_'.join(['-'.join(e) for e in sorted_entities])
+    # expected BIDSFile:
+    suffix = 'T1w'
+    extension = 'nii.gz'
+    expected_bids_file = BIDSFile(OrderedDict(sorted_entities), suffix, extension)
+
+    # entities in random order:
+    idcs = list(range(len(sorted_entities)))
+    shuffle(idcs)
+    shuffled_entities = [sorted_entities[i] for i in idcs]
+    shuffled_str = '_'.join(['-'.join(e) for e in shuffled_entities])
+
+    # Test __eq__ method.
+    # It should consider equal BIDSFiles with the same entities even in different order:
+    assert BIDSFile(OrderedDict(shuffled_entities), suffix, extension) == expected_bids_file
+
+    # Test __getitem__:
+    assert all([expected_bids_file[k] == v for k, v in shuffled_entities])
+
+    # Test filename parser and  __str__ method:
+    # Note: the __str__ method should return entities in the correct order
+    ending = '_T1w.nii.gz'  # suffix + extension
+    my_bids_file = BIDSFile.parse(shuffled_str + ending)
+    assert my_bids_file == expected_bids_file
+    assert str(my_bids_file) == expected_sorted_str + ending
+
+    ending = '.json'  # just extension
+    my_bids_file = BIDSFile.parse(shuffled_str + ending)
+    assert my_bids_file.suffix == ''
+    assert str(my_bids_file) == expected_sorted_str + ending
+
+    ending = '_T1w'  # just suffix
+    my_bids_file = BIDSFile.parse(shuffled_str + ending)
+    assert my_bids_file.extension is None
+    assert str(my_bids_file) == expected_sorted_str + ending
+
+    # Complain if entity 'sub' is not set:
+    with pytest.raises(ValueError) as e_info:
+        assert str(BIDSFile.parse('dir-reversed.json'))
+        assert 'sub-' in e_info.value
+
+    # Test set method:
+    # -for a new entity, just set it without a complaint:
+    my_bids_file['dir'] = 'AP'
+    assert my_bids_file['dir'] == 'AP'
+    # -for an existing entity, don't change it by default:
+    my_bids_file['echo'] = '2'
+    assert my_bids_file['echo'] == expected_bids_file['echo']  # still the original value
+    # -for an existing entity, you can overwrite it with "set":
+    my_bids_file.set('echo', '2')
+    assert my_bids_file['echo'] == '2'
+
     assert BIDSFile._known_entities[:2] == ['sub', 'ses']
     print(BIDSFile._known_entities)
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