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I was having an issue installing a package in Python 3, which installed properly in Python 2. This is the error message I got:
 Complete output from command python setup.py egg_info:
 Traceback (most recent call last):
 ... snip unhelpful traceback ...
 File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/configparser.py", line 423, in _interpolate_some
 "found: %r" % (rest,))
 configparser.Interpolation
 ----------------------------------------
 Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/12/p4ngkfbx2pnb1ln81csjb19c0000gn/T/pip-build-6xhgg5x6/nengo
This wasn't a super helpful error message. I managed to figure out that Python 3 (or setuptools?) was attempting to parse my ~/.pypirc file, which raised a configparser.InterpolationSyntaxError because my password contained a percent-sign. Configparser doesn't try to interpolate this string in Python 2, so it worked fine. As a workaround, I've changed my PyPI password, but this failure was quite surprising to me.
As for what I would expect to happen, I would not expect interpolation to happen if I don't want it to happen. All of the examples shown on in the docs use the syntax "%(key)s" or "${key}". If this is the only way to interpolate, then why is "%" ambiguous? Surely we can look forward one character, and if it's not "(" then "%" should be interpreted literally. Instead, configparser requires me to disambiguate with "%%". But, this would cause my string to be parsed incorrectly in Python 2's configparser, so I'm unable to make that change.