Expose factory.SKIP to omit a field from the generated object

[rodrigobnogueira]

Summary

Exports factory.SKIP, the sentinel Maybe already uses as its default branch. A field that resolves to it is left out of the arguments prepare_arguments builds, for every factory: a DictFactory omits the key, and a model factory falls back to the model's own default for that argument.

class ConfigFactory(factory.DictFactory):
    host = "localhost"
    debug = factory.SKIP

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

Closes #1140.

Changes

• Export SKIP from factory and document it with its own .. data:: SKIP entry next to Maybe.
• Transformer no longer passes SKIP to its transform function, which raised TypeError for functions such as str.upper. This also covers a wrapped declaration that resolves to SKIP, such as Transformer(Maybe("flag", "x"), transform=...) with the flag off.
• SKIP gets a repr and keeps its identity through copy, deepcopy and pickle, which the identity checks depend on.

[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