Strange behavior of .filename() method

[ewestern]

b = hydra.WritingBloomFilter(10000, 0.1, filename='/tmp/foo')
b.filename()
''
b.add('asd')
b.add('sdsds')
b.fdatasync()
b.filename()
''
b = hydra.WritingBloomFilter(10000, 0.1)
b.add('asd')
b.add('sdsds')
b.filename()
'e'

$ uname -a
Linux ip-* 4.4.0-1020-aws #29-Ubuntu SMP Wed Jun 14 15:54:52 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

$ cython --version
Cython version 0.27.3

$ python --version
Python 2.7.12

I'd like to be able to retrieve the path of the mmaped file once the bloom is closed.