oss_24_declarative_config_present_null_defaults

[ocelotl]

Closes #31

Declarative config: a present-but-null component (e.g. console: with no value) now builds that component with defaults instead of raising ConfigurationError; present-null is carried through conversion distinctly from an absent key (Linear OSS-24, finding L1).

Validation: scope contained to opentelemetry-configuration + changelog; full package suite 369 passed.

Linear issue: https://linear.app/dash0/issue/OSS-24/l1-declarative-config-present-but-null-components-fail-instead-of

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-24/l1-declarative-config-present-but-null-components-fail-instead-of (finding L1)
• Closes: oss_24_declarative_config_present_null_defaults #31 · Branch: oss_24_declarative_config_present_null_defaults · Commit: 9e7d6534b
• Validation: opentelemetry-configuration suite 369 passed; scope package + changelog.

[ocelotl]

📣 Public-facing draft — to be used for the upstream PR in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Fix declarative config present-but-null components to build with defaults

What

Make a present-but-null component value in declarative configuration build that component with defaults, instead of raising ConfigurationError.

Why

A present-but-null value (e.g. console: with no value) mapped to None during dict-to-dataclass conversion, indistinguishable from an absent key. Component factories select with value is not None, so a present-null component was treated as "not configured" and raised ConfigurationError.

Spec

configuration/sdk.md requires parsing to distinguish a missing key from a present-null one, and a present-null component MUST be created with all defaults (matching Java and Go, which bind the raw node and construct with defaults).

How

_convert_value is only invoked for keys actually present in the parsed mapping — absent keys fall through to dataclass field defaults — so a None value there always means present-null. It is now carried through as:

• a defaults-only dataclass instance for dataclass-typed fields (e.g. otlp_http: → OtlpHttpExporter() with all fields None, so the exporter reads its own defaults / env vars),
• an empty mapping for mapping-alias fields (e.g. the console exporter, typed dict[str, Any] | None), so the selecting factory still fires,
• None for primitives.
  Absent keys remain None, preserving "not configured" semantics. No factory-selection changes needed.

Tests

New unit tests in test_conversion.py (present-null dataclass → defaults instance; present-null mapping-alias console → {}; present-null primitive → None; absent → None; populated still converts) and factory-level tests in test_tracer_provider.py (present-null console builds a ConsoleSpanExporter; absent exporter type still raises). Full package suite: 369 passed.