__getitem__ from offsetmap.py for Medium article https://medium.com/p/a394fa48b671/

offsetmap-part.py

def __getitem__(self, path):
    origpath = path
    if '/' in path:
        prefix, item = path.rsplit('/',1) 
    else:
        prefix = ''
        item = path

    if self._chunk_re.match(item):
        offset = self._get_offset(prefix)
        if offset:
            # Apply offset
            chunks = [int(i) for i in item.split('.')]
            ochunks = [chunk + offset[i] for i,chunk in enumerate(chunks)]
            item = '.'.join(str(v) for v in ochunks)

    path = prefix + '/' + item if prefix else item
    return super().__getitem__(path)