Dynamic Expressions

As an alternative to fixed values, Schedy accepts so called expressions in schedule rules.

Expressions are a powerful way of expressing a value to be sent to actors dynamically in relation to anything you can think of. This power comes from the fact that expressions are just normal Python code which is evaluated at runtime. All expressions are pre-compiled at startup to make their later evaluation really performant.

Contents:

• Writing Expressions
• Examples
• Expression Helpers
• Postprocessing Results
• Result Markers

Security Considerations

It has to be noted that expressions are evaluated using Python’s exec() function. In general, this is not suited for code originating from a source you don’t trust completely, because such code can potentially execute arbitrary commands on your system with the same permissions and capabilities the AppDaemon process itself has. That shouldn’t be a problem for expressions you write yourself inside schedules.

This feature could however become problematic if an attacker somehow is able to emit events on your Home Assistant’s event bus. To prevent expressions from being accepted in the schedy_set_value event, processing of such expressions is disabled by default and has to be enabled explicitly by setting expressions_from_events: true in your Schedy configuration.