flake8 testing of https://github.com/bitcoin/bips on Python 3.6.3
$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
./bip-0069/bip-0069_examples.py:52:9: F821 undefined name 'bytearray_cmp'
return bytearray_cmp(output_tuple1[1], output_tuple2[1])
^
1 F821 undefined name 'bytearray_cmp'