Following system colour scheme Selected dark colour scheme Selected light colour scheme

Python Enhancement Proposals

PEP 685 – Comparison of extra names for optional distribution dependencies

Brett Cannon <brett at>
Paul Moore <p.f.moore at>
Discourse thread
Standards Track
Discourse post

Table of Contents


This PEP is a historical document. The up-to-date, canonical specifications are maintained on the PyPA specs page.

See the PyPA specification update process for how to propose changes.


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.


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. There is no other PyPA specification which outlines how extra names should be written or normalized 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 existing code, while being something tool authors can agree to following.

The issue of there being no consistent 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.


PEP 503 specifies how to normalize distribution names:

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

This collapses any run of the characters -, _ and . down to a single -. For example, --- . and __ all get converted to just -. This does not produce a valid Python identifier, per the core metadata 2.2 specification for extra names.

Setuptools 60 performs normalization via:

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

The use of an underscore/_ differs from PEP 503’s use of a hyphen/-, and it also normalizes characters outside of those allowed by PEP 508. Runs of . and -, unlike PEP 503, do not get normalized to one _, e.g. .. stays the same. To note, this is inconsistent with this function’s docstring, which does specify that all non-alphanumeric characters (which would include - and .) are normalized and collapsed.

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

Tzu-ping Chung on Python Discourse <


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. This will bring extra naming in line with that of the Name field. Because this changes what is considered valid, it will lead to a core metadata version increase to 2.3.

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 generating metadata MUST raise an error if an invalid extra name is provided as appropriate for the specified core metadata version. If a project’s metadata specifies an older core metadata version and the name would be invalid with newer core metadata versions, tools reading that metadata SHOULD warn the user. Tools SHOULD warn users when an invalid extra name is read and SHOULD ignore the name to avoid ambiguity. Tools MAY raise an error instead of a warning when reading an invalid name, if they so desire.

Backwards Compatibility

Moving to PEP 503 normalization and PEP 508 name acceptance 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 instances when considering all extras names on PyPI, valid or not (not just those within a single package) while only looking at valid names leads to only 3 clashes:

  • dev-test: dev_test, dev-test, dev.test
  • dev-lint: dev-lint, dev.lint, dev_lint
  • 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 diminish over time.

Paul Moore on Python Discourse

Security Implications

It is possible that for a distribution that has conflicting extra names, a tool ends up installing dependencies 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 module 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.

Transition Plan

There is a risk that a build tool will produce core metadata conforming to version 2.3 and thus this PEP but which is consumed by a tool that is unaware of this PEP (if that tool chooses to attempt to read a core metadata version it does not directly support). In such a case there is a chance that a user may specify an extra using an non-normalized name which worked previously but which fails now.

As such, consumers of this PEP should be prioritized more than producers so that users can be notified that they are specifying extra names which are not normalized (and thus may break in the future).

Rejected Ideas

Using setuptools 60’s normalization

Initially, this PEP proposed using setuptools safe_extra() for normalization to try to minimize backwards-compatibility issues. However, after checking various wheels on PyPI, it became clear that standardizing all naming on PEP 508 and PEP 503 semantics was easier and better long-term, while causing minimal backwards compatibility issues.

Open Issues



Last modified: 2022-10-07 15:29:13 GMT