feat: Add MISSING sentinel to exclude keys from DictFactory

[rodrigobnogueira]

Related Issues

Closes #1140 - Sentinel object for excluding items in DictFactory

Summary

This PR adds a MISSING sentinel value for DictFactory that allows excluding specific keys from the generated dictionary.

Motivation

When generating dictionary payloads (e.g., API responses), there are cases where you want to conditionally exclude certain keys:

• Optional fields that should only appear under certain conditions
• Fields that should be absent rather than null
• Dynamic payloads where presence/absence of keys matters

Changes

MISSING Sentinel

Features:

• Singleton sentinel with falsy boolean value
• Works with Transformer declarations
• Can be conditionally returned from LazyFunction
• Overridable at call time to include the key

import factory

class ResponseFactory(factory.DictFactory):
    name = factory.Faker('name')
    email = factory.MISSING  # This key will be excluded

result = ResponseFactory()
# Result: {'name': 'John Doe'}
# Note: 'email' key is not present

# Conditional exclusion
class ConditionalFactory(factory.DictFactory):
    field = factory.LazyFunction(lambda: factory.MISSING if condition else "value")

# Override to include
result = ResponseFactory(email="user@example.com")
# Result: {'name': 'John Doe', 'email': 'user@example.com'}

Testing

• 9 new tests covering the feature
• Full test suite: 448 passed
• Covers: basic functionality, override behavior, Transformer interaction, LazyFunction usage

Checklist

• Tests pass locally
• New tests added for new functionality
• Follows existing code style

[chriswyatt]

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