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# Modifying events in an application using `jupyter_events`

If you’re deploying a configurable application that uses Jupyter Events to emit events, you can extend the application’s event logger to modify/mutate/redact incoming events before they are emitted. This is particularly useful if you need to mask, salt, or remove sensitive data from incoming event.

To modify events, define a callable (function or method) that modifies the event data dictionary. This callable must follow an exact signature (type annotations required):

    def my_modifier(schema_id: str, data: dict) -> dict:
        ...

The return value is the mutated event data (dict). This data will be validated and emitted after it is modified, so it still must follow the event’s schema.

Next, add this modifier to the event logger using the `.add_modifier` method:

    logger = EventLogger()
    logger.add_modifier(my_modifier)

This method enforces the signature above and will raise a `ModifierError` if the signature does not match.