Thanks for the quick response! I can see the use case for using interpolation in .pypirc. Unfortunately for me, I push releases for both Python 2 and Python 3, so having the double percent sign will cause problems for me on Python 2.

The exception that's being raised is at line 442 (https://hg.python.org/cpython/file/default/Lib/configparser.py#l442). The message itself seems to have the right information, but setuptools seems to mangle it  (perhaps InterpolationSyntaxError needs a __repr__ or __str__?)