Method invoke

sdk/python/src/lib.rs:3487–3522

(
        &self,
        prev: Option<&RustStepOutcome>,
        item: &serde_json::Value,
    )

impl PythonPipelineStage {
    fn invoke(
        &self,
        prev: Option<&RustStepOutcome>,
        item: &serde_json::Value,
    ) -> Option<RustAgentStepSpec> {
        pyo3::Python::with_gil(|py| {
            let result = (|| -> PyResult<Option<RustAgentStepSpec>> {
                let json_mod = py.import("json")?;
                let previous = match prev {
                    Some(o) => {
                        let s = serde_json::to_string(o)
                            .map_err(|e| PyValueError::new_err(e.to_string()))?;
                        json_mod.call_method1("loads", (s,))?
                    }
                    None => py.None().into_bound(py),
                };
                let item_str = serde_json::to_string(item)
                    .map_err(|e| PyValueError::new_err(e.to_string()))?;
                let item_py = json_mod.call_method1("loads", (item_str,))?;
                let ctx = PyDict::new(py);
                ctx.set_item("previous", previous)?;
                ctx.set_item("item", item_py)?;
                let ret = self.callback.call1(py, (ctx,))?;
                let bound = ret.bind(py);
                if bound.is_none() {
                    return Ok(None);
                }
                let spec_json: String = json_mod.call_method1("dumps", (bound,))?.extract()?;
                serde_json::from_str::<RustAgentStepSpec>(&spec_json)
                    .map(Some)
                    .map_err(|e| PyValueError::new_err(format!("invalid step spec: {e}")))
            })();
            // Fail-closed: any exception → stop this chain.
            result.unwrap_or(None)
        })
    }
}

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