I created https://github.com/python/cpython/pull/845 to change struct.Struct.format type to str (Unicode).

struct.Struct() accepts bytes and str format strings, so it's not really a backward incompatible change.

It's just a minor enhancement to help development:

$ ./python
Python 3.7.0a0 (heads/master-dirty:b8a7daf, Mar 27 2017, 13:02:20) 
>>> print(struct.Struct('hi').format)
hi


Without the patch:

haypo@selma$ python3
Python 3.5.2 (default, Sep 14 2016, 11:28:32) 
[GCC 6.2.1 20160901 (Red Hat 6.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> print(struct.Struct('hi').format)
b'hi'

haypo@selma$ python3 -bb
Python 3.5.2 (default, Sep 14 2016, 11:28:32) 
>>> import struct
>>> print(struct.Struct('hi').format)
Traceback (most recent call last):
  ...
BytesWarning: str() on a bytes instance