Originally posted by Russell Keith-Magee on Twitter: https://twitter.com/freakboy3742/status/812609675283681280 Reproduced successfully on Python 3.5.2 >>> x = bytearray(1) >>> y = {} >>> x % y Python3.5: bytearray(b'') Python3.6: bytearray(b'\x00') also: >>> bytearray([0,1]) % {} # same with [], (), range(42) bytearray(b'') >>> bytearray([1,0]) % {} bytearray(b'\x01')

Since I was named dropped; it's worth pointing out that this has evidently been fixed - intentionally or otherwise - in 3.6. It wasn't an issue in 3.4 and earlier because __mod__ wasn't implemented for bytestrings.

Indeed, there is an inconsistency between bytes and bytearray in 3.5: >>> b'a\0b' % () b'a\x00b' >>> bytearray(b'a\0b') % () bytearray(b'a') It was unintentionally fixed in issue25399. Proposed patch fixes the issue.

New changeset 277b36596a54 by Serhiy Storchaka in branch '3.5': Issue #29073: bytearray formatting no longer truncates on first null byte. https://hg.python.org/cpython/rev/277b36596a54 New changeset 9b77e3a586b0 by Serhiy Storchaka in branch '3.6': Issue #29073: Added a test for bytearray formatting with null byte. https://hg.python.org/cpython/rev/9b77e3a586b0 New changeset 82bfdf599e24 by Serhiy Storchaka in branch 'default': Issue #29073: Added a test for bytearray formatting with null byte. https://hg.python.org/cpython/rev/82bfdf599e24