build_meta doesn't use a fresh dist directory, which causes ValueError unpacking tuple

[pganssle]

If you use pip install . on a source directory that already has a dist, the existing directory structure is used when preparing the wheel. This causes an exception if an existing wheel with a different name exists, because build_meta assumes there's only one wheel in dist.

Here's a script to create a MWE repo:

#!/usr/bin/bash

mkdir /tmp/demo_dist_517
cd /tmp/demo_dist_517
echo "from setuptools import setup; setup()" > setup.py
echo "0.0.1" > VERSION
cat > setup.cfg << EOF
[metadata]
name = foo
version = file: VERSION
EOF
cat > pyproject.toml << EOF
[build-system]
requires = ["setuptools", "wheel"]
build-backend = "setuptools.build_meta"
EOF

At this point your repo looks like this:

$ tree
├── pyproject.toml
├── setup.cfg
├── setup.py
└── VERSION

Create a wheel in dist, then change the version:

pip wheel . --no-deps -w dist
echo "0.0.2" > VERSION

Now try to create a wheel from the repo:

pip wheel . -w dist

This will trigger an error in build_meta:
File "/tmp/pip-build-env-plomixa1/overlay/.../setuptools/build_meta.py", line 157, in _file_with_extension
file, = matching
ValueError: too many values to unpack (expected 1)
Building wheel for foo (PEP 517) ... error
Failed building wheel for foo

This is pretty easy to work around, obviously, since the user can just remove dist before invoking any pip commands, but it might be better to do the wheel build in a clean directory if possible.

[pganssle]

I could be wrong, but I think we can somewhat trivially fix this by changing build_meta.build_wheel to directly use --dist-dir=wheel_directory, so:

    sys.argv = sys.argv[:1] + ['bdist_wheel'] + \
        config_settings["--global-option"]
    _run_setup()
    if wheel_directory != 'dist':
        shutil.rmtree(wheel_directory)
        shutil.copytree('dist', wheel_directory)

Would turn to:

    sys.argv = sys.argv[:1] + ['bdist_wheel', '--dist-dir=%s' % wheel_directory] + \
        config_settings["--global-option"]
    _run_setup()

I'm not sure what happens if --dist-dir is specified twice, so maybe we want to detect if --dist-dir or -d is already in config_settings['--global-option'] and if so use the old behavior. Though that might not matter, since right now using --dist-dir will fail in some other weird way.

[pradyunsg]

FYI pypa/pip@51086b3

[pganssle]

@pradyunsg Is this causing major problems? I put it in my mental backlog and left as a possible sprint item just because it seemed like it would only affect maintainers who leave build artifacts around in their source directories. If it's causing real problems for other people, I'll at least bump it up in priority from "minor".

[pradyunsg]

Nope -- this is pretty minor IMO.

I wanted to note here that this showed up while testing pip wheel when that broke due to PEP 517 changes on pip's side. Should've been less terse, sorry!

[florisla]

Hi, can the title of this issue be changed to include "ValueError: too many values to unpack" ?
That would make it easier to Google when you encounter the bug -- like I did recently.

This bug is easily triggered in this (not very far-fetched) scenario:

• Use black in your project, so start using pyproject.toml to configure it.
• Build a wheel (e.g. when releasing a package version).
• Change version number (e.g. moving to the next beta).
• Use tox or nox (or any tool which will trigger a wheel build without cleaning up first).