fix: correct some signatures and fix PyObserver implementation

python/laddu/__init__.pyi

@@ -14,7 +14,7 @@ from . import amplitudes, data, utils
 
 class Observer(metaclass=ABCMeta):
     @abstractmethod
-    def callback(self, step: int, status: Status, expression: Expression) -> tuple[Status, Expression, bool]: ...
+    def callback(self, step: int, status: Status) -> tuple[Status, bool]: ...
 
 __version__: str
 

python/laddu/likelihoods/__init__.pyi

@@ -3,7 +3,7 @@ from typing import Literal
 import numpy as np
 import numpy.typing as npt
 
-from laddu.amplitudes import Manager
+from laddu.amplitudes import Expression, Manager
 from laddu.data import Dataset
 
 class LikelihoodID:
@@ -37,7 +37,7 @@ class LikelihoodEvaluator:
 
 class NLL:
     parameters: list[str]
-    def __init__(self, manager: Manager, ds_data: Dataset, ds_mc: Dataset) -> None: ...
+    def __init__(self, manager: Manager, ds_data: Dataset, ds_mc: Dataset, expression: Expression) -> None: ...
     def activate(self, name: str | list[str]) -> None: ...
     def activate_all(self) -> None: ...
     def deactivate(self, name: str | list[str]) -> None: ...

src/python.rs

@@ -1473,7 +1473,7 @@ impl Observer<Float, ()> for crate::python::laddu::PyObserver {
                 .extract::<crate::python::laddu::Status>()
                 .unwrap()
                 .0;
-            let result = res_tuple.get_item(2).unwrap().extract::<bool>().unwrap();
+            let result = res_tuple.get_item(1).unwrap().extract::<bool>().unwrap();
             (new_status, result)
         });
         *status = new_status;