Contents

• Abstract
• Motivation
• Rationale
• Specification
• Backwards Compatibility
• Security Implications
• How to Teach This
• Reference Implementation
• Rejected Ideas
  • Normalize names according to PEP 503
• Open Issues
• Copyright

Abstract

This PEP specifies how to normalize distribution extra names when performing comparisons. This prevents tools from either failing to find an extra name, or accidentally matching against an unexpected name.

Motivation

The Provides-Extra core metadata specification states that an extra’s name “must be a valid Python identifier”. PEP 508 specifies that the value of an extra marker may contain a letter, digit, or any one of ., -, or _ after the initial character. Otherwise, there is no other PyPA specification which outlines how extra names should be written or normalization for comparison. Due to the amount of packaging-related code in existence, it is important to evaluate current practices by the community and standardize on one that doesn’t break most code, while being something tool authors can agree to following.

The issue of there being no standard was brought forward by an initial discussion noting that the extra adhoc-ssl was not considered equal to the name adhoc_ssl by pip 22.

Rationale

PEP 503 specifies how to normalize distribution names:

re.sub(r"[-_.]+", "-", name).lower()

This collapses any run of the substitution character down to a single character, e.g. --- gets collapsed down to -. This does not produce a valid Python identifier as specified by the core metadata 2.2 specification for extra names.

Setuptools 60 does normalization via:

re.sub('[^A-Za-z0-9.-]+', '_', name).lower()

The use of an underscore/_ differs from PEP 503’s use of a hyphen/-. Runs of _, unlike PEP 503, do not get collapsed, e.g. ___ stays the same.

For pip 22, its “extra normalisation behaviour is quite convoluted and erratic” [pip-erratic], and so its use is not considered.

[pip-erratic]

https://discuss.python.org/t/what-extras-names-are-treated-as-equal-and-why/7614/10?

Specification

When comparing extra names, tools MUST normalize the names being compared using the semantics outlined in PEP 503 for names:

re.sub(r"[-_.]+", "-", name).lower()

The core metadata specification will be updated such that the allowed names for Provides-Extra matches what PEP 508 specifies for names. As this is a superset of what is currently allowed by the core metadata 2.2 specification, it allows for a loosening of the naming requirements. It will also bring extra naming in line with that of the Name field.

For tools writing core metadata, they MUST write out extra names in their normalized form. This applies to the Provides-Extra field and the extra marker when used in the Requires-Dist field.

Tools generating metadata MUST raise an error if a user specified two or more extra names which would normalize to the same name. Tools SHOULD warn users when an invalid extra name is read.

Backwards Compatibility

Moving to PEP 503 normalization and PEP 508 name acceptance, it allows for all preexisting, valid names to continue to be valid.

Based on research looking at a collection of wheels on PyPI [pypi-results], the risk of extra name clashes is limited to 73 clashes when considering even invalid names, while only looking at valid names leads to only 3 clashes:

1. dev-test: dev_test, dev-test, dev.test
2. dev-lint: dev-lint, dev.lint, dev_lint
3. apache-beam: apache-beam, apache.beam

By requiring tools writing core metadata to only record the normalized name, the issue of preexisting, invalid extra names should be diminished over time.

[pypi-results]

https://discuss.python.org/t/pep-685-comparison-of-extra-names-for-optional-distribution-dependencies/14141/17?u=brettcannon

Security Implications

It is possible that for a distribution that has conflicting extra names, a tool ends up installing distributions that somehow weaken the security of the system. This is only hypothetical and if it were to occur, it would probably be more of a security concern for the distributions specifying such extras names rather than the distribution that pulled them in together.

How to Teach This

This should be transparent to users on a day-to-day basis. It will be up to tools to educate/stop users when they select extra names which conflict.

Reference Implementation

No reference implementation is provided aside from the code above, but the expectation is the packaging project will provide a function in its packaging.utils that will implement extra name normalization. It will also implement extra name comparisons appropriately. Finally, if the project ever gains the ability to write out metadata, it will also implement this PEP.

Rejected Ideas

Open Issues

N/A

Copyright

This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.