feat: Export SKIP sentinel for excluding keys from DictFactory

[rodrigobnogueira]

Summary

Exports factory.SKIP to the public API, allowing users to exclude keys from DictFactory output.

Closes #1140

Motivation

When using DictFactory, there was no documented way to conditionally exclude keys from the generated dictionary. The SKIP sentinel was already available internally but not exposed to users.

Changes

• Export SKIP from factory.declarations to the top-level factory package
• Add check in Transformer.evaluate_pre to bypass transformation when value is SKIP
• Add comprehensive tests for SKIP behavior with DictFactory
• Add documentation example in reference.rst
• Add ChangeLog entry

Usage

import factory

class ConfigFactory(factory.DictFactory):
    host = "localhost"
    port = 8080
    debug = factory.SKIP  # This key will be omitted

ConfigFactory()  # {'host': 'localhost', 'port': 8080}
ConfigFactory(debug=True)  # {'host': 'localhost', 'port': 8080, 'debug': True}

Testing

• 8 new tests covering the feature in tests/test_skip.py
• Full test suite passes (verified locally)
• Covers: basic exclusion, per-instance override, skipping all fields, LazyFunction compatibility, singleton behavior, falsy nature, and Transformer interaction

Checklist

• Tests pass locally
• New tests added for new functionality
• Documentation added reference.rst
• ChangeLog updated
• Follows existing code style

[chriswyatt]

I tested this in one of my projects and it seems to work fine

[chriswyatt]

I've just realised that the SKIP declaration might already do what I want, though it's not obvious, as the documentation only shows it being used with factory.Maybe. I'm also not yet sure if SKIP would work with all sorts of edge cases (e.g. transformers, class inheritance with overrides, traits, etc.).

[rodrigobnogueira]

The use of SKIP looks promising indeed. SKIP already exists internally in factory.declarations.SKIP and is already handled in prepare_arguments. However, SKIP isn't currently exported at the package level—you can't do factory.SKIP directly. It's only used internally for Maybe and Trait defaults.

The semantics of both SKIP and MISSING seem consistent: 'omit this attribute'

we could:

Export SKIP from factory/init.py
Add documentation showing it can be used standalone (not just with Maybe)

[chriswyatt]

Yes, that sounds good. And I guess in cases where you are using, say, a dataclass as a model, SKIP makes more sense than MISSING, as you will be getting a default value instead.

[rodrigobnogueira]

The MISSING constant was removed and only a small number of adjustments were necessary to use SKIP instead. The previous tests for MISSING have been updated for SKIP. It was really a great insight @chriswyatt . thanks!

• Exported SKIP from factory/init.py (now part of public API)
• Removed all MISSING sentinel code (26 lines removed)
• Added SKIP check to Transformer.evaluate_pre to prevent transformation errors
• Renamed test_missing.py → test_skip.py and updated all 8 test methods