Treat every potentially-null/nullable config subsection and external payload field as untrusted: normalize it before accessing deeper keys or calling methods.

Rules

• When reading nested config, guard for both missing keys and null subsections (common in YAML):
  • Prefer section = cfg.get("x") or {} before section.get(...).
• When reading config values that might be the wrong type (e.g., mapping expected but boolean provided), validate the shape:
  • Use isinstance(value, dict) (or equivalent) before nested .get().
• When using external event fields, validate types before string operations (e.g., startswith):
  • Check isinstance(field, str).
• Ensure annotation types referenced in runtime-evaluated modules are imported (avoid NameError from Optional/Any).

Example

def pick_language(stt_config: dict) -> str | None:
    stt_config = stt_config or {}
    groq_cfg = stt_config.get("groq") or {}          # handles groq: null
    local_cfg = stt_config.get("local") or {}
    return groq_cfg.get("language") or local_cfg.get("language")

def handle_matrix_location(payload: dict) -> None:
    geo_uri = payload.get("geo_uri")
    if not isinstance(geo_uri, str):
        return  # or safely log/drop
    if geo_uri.startswith("geo:"):
        ...

This prevents crashes like AttributeError: 'NoneType' object has no attribute 'get' and AttributeError/TypeError from calling string methods on non-strings.