binhex.binhex() creates files with mixed line endings in Python 3. The header line '(This file must be converted with BinHex 4.0)' is separated from the data by LFs, but the data is split for lines by CRs.

>>> import binhex
>>> with open('inp', 'wb') as f: f.write(bytes(range(256)))
... 
256
>>> binhex.binhex('inp', 'outp')
>>> for line in open('outp', 'rb').read().splitlines(True): print(repr(line))
... 
b'(This file must be converted with BinHex 4.0)\r'
b'\r'
b':!fPZF!!rN!J!N!3"!*!&*VF!!3)$"!8\'"`J*#JX-$3i2%"%5%a39&KFB\'4SE("d\n'
b'H(b!K)L-N*5BR+#NU+b`Y,Lm`-6)c0$8f0cJj1MXm26ir3%&#3d4&4NG)58T,6%e\n'
b'16e"48P0899CA@&PD@eaGAPpJB@*MC\'9QCfKTDQYXE@j[F(&bFh4eGRGiHATlI(e\n'
b'qIi#"JS1%KBD(L)Q+Lib0MSq3!*\'5Nj59PTHBQCUER*fHRk#KSU1NTDDRU+QUUkb\n'
b'YVUq`XE+cY,@fYlLjZVZm[Ekr`-(#`m6&aXI)bFV,c-h1cp$4dY28eGEAf0RDfpc\n'
b'GhYrJiH,Mj1AQjqMTkZ[XlHl[m2(bmr6ep[IiqIVlr2hqrhj9!!!:\n'

In Python 2 the output file was file object usually opened in text mode. Newline characters were translated to platform-depending line endings: CRLF on Windows, LF on classic Mac OS. In Python 2 the output file is binary stream that doesn't do any newline translations.

The last related commit is 34a042d301d6ab88645046a6dfa6c38265ca4b39 with Guido's message "This is the last time I fix binhex.  If it breaks again it goes in the dustbin."

I've added a PR that changes the line ending to "\r" in encoded data.

Not very useful now that binhex is deprecated, but this allows us to close this issue.

New changeset 2165cea548f961b308050f30d1f042a377651d44 by Ronald Oussoren in branch 'master':
bpo-29566: binhex.binhex now consitently writes MacOS 9 line endings. (GH-23059)
https://github.com/python/cpython/commit/2165cea548f961b308050f30d1f042a377651d44

New changeset 39a56e55231be00d52fa183fcd2b7d88619ced4b by Miss Skeleton (bot) in branch '3.8':
[3.8] bpo-29566: binhex.binhex now consitently writes MacOS 9 line endings. (GH-23059) (GH-23070)
https://github.com/python/cpython/commit/39a56e55231be00d52fa183fcd2b7d88619ced4b

New changeset 3defcbac2c9ff4306ed5b7fb37d12637eb188306 by Miss Skeleton (bot) in branch '3.9':
[3.9] bpo-29566: binhex.binhex now consitently writes MacOS 9 line endings. (GH-23059) (GH-23071)
https://github.com/python/cpython/commit/3defcbac2c9ff4306ed5b7fb37d12637eb188306