We discovered this accidentally by way of zopefoundation/zc.relation#4 (comment): if you pass a string containing newlines to the description argument of setup(), setuptools will generate a malformed PKG-INFO.

To reproduce:

# setup.py
from setuptools import setup
setup(
    name='test-package',
    version='0.1',
    author='Blah Blah',
    author_email='blah@example.com',
    description='description\n\n',
    py_modules=['blah'],
)

(The contents of blah.py do not matter, but the file should exist.)

Run python setup.py sdist and the inspect test_package.egg-info/PKG-INFO. For me, with setuptools 39.1.0, it looks like this:

Metadata-Version: 1.0
Name: test-package
Version: 0.1
Summary: description


Home-page: UNKNOWN
Author: Blah Blah
Author-email: blah@example.com
License: UNKNOWN
Description: UNKNOWN
Platform: UNKNOWN

The extra newlines lead tools to treat the rest of the PKG-INFO as a long_description.

I would expect setuptools to complain about the newlines in the description field, or at least escape them properly (i.e. prepend whitespace, like it does for the long_description field).