Valid toml syntax causes black to crash

[nat-n]

Describe the bug

I have a pyproject.toml for configuring various other tools (not black), which includes a list with mixed type contents, as explicitly support by the v1.0.0 of the toml spec.

When I run black it crashes with a cryptic traceback from inside the toml library.

To Reproduce

mkdir new_project
cat <<EOT > ./new_project/pyproject.toml
# this example is from the toml spec
contributors = [
  "Foo Bar <foo@example.com>",
  { name = "Baz Qux", email = "bazqux@example.com", url = "https://example.com/bazqux" }
]
EOT
cd new_project
black .

Expected behavior

It shouldn't crash.

Environment (please complete the following information):

• Version: black, version 21.5b1
• OS and Python version: MacOs Python 3.8.8

---

Since the presently used "toml" library explicitly only supports up to version 0.5.0 of the spec (from about two years ago), I'd suggest switching to tomlkit, which supports the current toml spec (officially up to 1.0.0rc1 but nothing has materially changed since then).

I'd be happy to put together a PR if this approach is deemed acceptable in principle.

[JelleZijlstra]

Seems reasonable. It looks like pip itself still uses toml though, and in practice compatibility with other Python tools that use pyproject.toml is probably more important for us than compatibility with the toml standard. There's some related discussion in https://discuss.python.org/t/adopting-recommending-a-toml-parser/4068.

[cooperlees]

tomli looks promising. Hope it can get into the stdlib so toml can become "real" in python as a low level config file format. Thanks for raising this interesting issue. For now tho I think you're stuck using 0.5.0 toml for pyproject.toml as I'd like us to follow what pip does too.

[nat-n]

I see, yes it seems pip has the same issue. I'd argue there's still value in supporting the latest toml syntax, even if pip does not yet. In principle it should be a non breaking change, though I guess the bottom line is that any divergence presents a risk of unexpected consequences?

[hukkin]

I made an issue in pip issue tracker about this: pypa/pip#10034

Let's see if they have willingness to upgrade to a TOML v1.0.0 compatible parser.

Edit: This also led to a proposal to change PEP 518 so that it includes a TOML spec version for pyproject.toml.