Python 3.7.0a0 (default:be70d64bbf88, Dec  1 2016, 21:21:25)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from array import array
>>> a = array('B', [1, 2])
>>> b'%b' % a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: %b requires bytes, or an object that implements __bytes__, not 'array.array'
>>> m = memoryview(a)
>>> b'%b' % m
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: %b requires bytes, or an object that implements __bytes__, not 'memoryview'

Accorfing to documentation [1] objects that follow the buffer protocol should be supported.  Both array.array and memoryview follow the buffer protocol.

[1]: https://docs.python.org/3/library/stdtypes.html#printf-style-bytes-formatting


See also issue 20284 and PEP 461.