Dbt Helpers Nullable Testing

dbt-helpers-nullable-testing

Purpose

To ensure fast, deterministic, and reliable testing without the brittleness of standard mocking libraries. This skill implements the "Functional Core, Imperative Shell" philosophy using the Nullable Infrastructure pattern.

Core Principles

1. No standard mocks: Avoid unittest.mock for infrastructure (I/O, DB, CLI).
2. Nullable Adapters: Real implementations with an "off switch" or in-memory substitute.
3. Internal State Tracking: Adapters should track their output/actions for state-based assertions.

Implementation Patterns

1. In-memory Substitutes

• Use DuckDB in-memory for catalog clients.
• Use a dictionary-based MemoryFileSystem for file I/O.
• Use a list-based BufferEmitter for CLI output.

2. Nullable Constructor

Each infrastructure class should provide a .create_null() factory method:

class CatalogClient(ABC):
    @classmethod
    def create_null(cls, metadata: dict = None):
        return NullCatalogClient(metadata)

Instructions

When writing unit tests

1. Use the .create_null() version of the adapter.
2. Provide pre-configured state to the nullable adapter (e.g., CatalogClient.create_null(tables=[...])).
3. Assert on the results of the logic, or check the adapter's internal state if necessary.

When implementing a new adapter

1. Define the interface (Protocol/ABC) in the SDK.
2. Implement the production version.
3. Implement the Nullable version (usually in the same file or a testing.py module).

References

• ADR 0006: Nullable Infrastructure Testing Strategy
• Testing Without Mocks (James Shore)

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