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Unit contexts

Unit contexts let a plugin provide metadata-dependent unit conversions without adding domain-specific traits to core objects.

For example, an NMR plugin can convert between ppm and Hz using acquisition frequency metadata stored on a coordinate. The plugin registers a context setup function, an optional predicate, and an optional argument extractor:

def register_unit_contexts(self) -> list[dict]:
    return [
        {
            "name": "nmr",
            "func": set_nmr_context,
            "predicate": applies_to_coord,
            "argument_extractor": get_larmor_frequency,
            "description": "NMR ppm/frequency conversion context",
        },
    ]

The core asks the registry for an applicable context during unit conversion. If no plugin context applies, normal Pint conversion is used.