Version "attr" method in setup.py builds dist names incorrectly

Hello,

Currently, the "Single-sourcing the package version" page provides several methods for storing a package's version in one location. The first includes a "modern" technique as of setuptools 46.4.0, describing this mechanism for use in setup.cfg:

[metadata]
version = attr: package.__version__

I was pleasantly surprised to find the setuptools.setup() call would also accept (read: not halt in an error state) this string in its version= parameter. However, I found none of the distribution files build with the correct name embedded.

I have set up a repository to test this issue here.

Setup

Given a package example_pkg, the setup.py program includes these lines:

setuptools.setup(
    name="example_pkg",
    version="attr: example_pkg.__version__",
  # ...
)

A wheel is built with this command:

python3 setup.py bdist_wheel

Expected output

I expect the generated file to be dist/example_pkg-0.0.1-py3-none-any.whl. This is the effect when the setup() call is written like this:

setuptools.setup(
    name="example_pkg",
    version="0.0.1",
  # ...
)

Current output

The output file is currently errantly named dist/example_pkg-attr_.example_pkg._version_-py3-none-any.whl.

Reproduction instructions

1. Clone the above linked repository.
2. Run make. The errant file is in the directory using_setup_py/dist. A functioning example of the same version retrieval syntax is in using_setup_cfg/.

As a last comment, it seems a bit ironic that the configuration file currently has more programmatic access to code variables than the setup script. Shouldn't these two methods behave the same? Or, is there a specification somewhere I've missed?

Start by cloning the linked reproduction repository and running make; compare using_setup_py/ with the functioning using_setup_cfg/ example, especially their setup.py and setup.cfg files. Reproduce the wheel built by python3 setup.py bdist_wheel and trace why its filename differs from the expected example_pkg-0.0.1-py3-none-any.whl; done means the setup.py version form produces the correct distribution name.