Value resolution¶

Reading a variable follows a fixed order in the active context: an explicitly stored value first, then a fresh factory result, then a deep copy of the declared default. A variable without any value raises LookupError unless get(default) supplies a call-site fallback.

Every source passes through the field converter: environment strings, set()/override() values, declared defaults, and factory results. A converter failure is therefore visible at the operation that introduced the value rather than deferred to a later consumer.

Declared defaults are copied when materialized, so mutating a default list in one context does not mutate the source default or another context's fresh value. This copying rule does not apply to values manually installed with set().