Inconsistency in _ -> - normalization

[Conchylicultor]

I'm using a PEP 621 toml using flit 3.5 & pip 21.3.1:

My toml has self-refering optional-dependencies:

[build-system]
requires = ["flit_core >=3.5,<4"]
build-backend = "flit_core.buildapi"

[project]
name = "etils"

[project.optional-dependencies]
array_types = ["numpy"]
all = ["etils[array_types]"]

When running pip install -e .[all], this logs:

WARNING: etils 0.2.0 does not provide the extra 'array-types'

It looks like "etils[array_types]" gets normalized to [array-types], but not array_types = ["numpy"].

Manually replacing array_types = ["numpy"] -> array-types = ["numpy"] solve the issue.

But this behavior feels inconsistent.

[takluyver]

PEP 508, which defines the syntax for specifying extras, allows -_. in the name of an extra, and doesn't specify any normalisation rule, which suggests that array_types and array-types are two separate extras. I'd guess that pip normalises them, probably like distribution names (rule in PEP 503). I don't know if there's any other spec that says this is the right thing to do, or if it's just following something that setuptools/easy_install have done without it ever being written in any spec.

The core metadata spec says that an extra name "must be a valid Python identifier", which implies that array-types is not even a legal value. That conflicts with PEP 508, so at least one of the specs must be wrong. 🤷

[pradyunsg]

Here's what flit generates (in the METADATA file, that pip reads from):

Requires-Dist: installer[array_types] ; extra == "all"
Requires-Dist: numpy ; extra == "array_types"
Provides-Extra: all
Provides-Extra: array_types

This is definitely a pip bug.

[Conchylicultor]

Thanks for the fast answer. Closing this. Will open a new issue in pip.

[Conchylicultor]

As follow up, it seems the inconsistency issue is discussed in https://discuss.python.org/t/7614