New submission from sorin:

The ConfigParser issue with % (percent) is taking huge proportions because it 
does have serious implications downstream. One such example is the fact that in 
breaks virtualenv in such way the if you create a virtual env in a path that 
contains percent at some point you will endup with a virtualenv where you can 
install only about 50% of existing python packages (serious ones like numpy or 
pandas will fail to install or even to perform a simple python setup.py 
egg_info on them).

This is related to distutils which is trying to use the ConfigParser to load 
the python PATH (which now contains the percent). 

Switching to RawConfigParser does resolve the problem but this seems like an 
almost impossible attempt because the huge number of occurrences in the wild.

You will find the that only class that is able to load a value with percent 
inside is RawConfigParser and I don't think that this is normal.

Here is some code I created to exemplify the defective behaviour:
https://github.com/ssbarnea/test-configparser/blob/master/tests/test-configparser.py#L21

The code is executed by Travis with multiple version of python, see one result 
example: https://travis-ci.org/ssbarnea/test-configparser/jobs/126145032

My personal impression is that the decision to process the % (percent) and to 
change the behaviour between Python 2 and 3 was a very unfortunate one. Ini/Cfg 
file specification does not specify the percent as an escape character. 
Introduction of the %(VAR) feature sounds more like bug than a feature in this 
case.

----------
components: Distutils
messages: 264402
nosy: dstufft, eric.araujo, sorin
priority: normal
severity: normal
status: open
title: Default ConfigParser in python is not able to load values habing percent 
in them
type: compile error
versions: Python 3.4, Python 3.5

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue26872>
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