TYP: accept scalar-returning IRR selectors

numpy_financial/_financial.py

@@ -1007,7 +1007,7 @@ def rate(
     return rn
 
 
-def _irr_default_selection(eirr: npt.NDArray[np.float64]) -> npt.NDArray[np.float64]:
+def _irr_default_selection(eirr: npt.NDArray[np.float64]) -> np.float64:
     """ default selection logic for IRR function when there are > 1 real solutions """
      # check sign of all IRR solutions
     same_sign = np.all(eirr > 0) if eirr[0] > 0 else np.all(eirr < 0)
@@ -1029,7 +1029,7 @@ def _irr_default_selection(eirr: npt.NDArray[np.float64]) -> npt.NDArray[np.floa
 
 _SelectionFunc: TypeAlias = Callable[
     [npt.NDArray[np.float64]],
-    npt.NDArray[np.float64],
+    float | np.floating[Any],
 ]
 
 

typing_tests/irr_selection.py

@@ -0,0 +1,29 @@
+"""Static IRR selector contract, checked by the repository-wide mypy job."""
+import numpy as np
+import numpy.typing as npt
+
+import numpy_financial as npf
+
+
+def select_python_float(candidates: npt.NDArray[np.float64]) -> float:
+    return float(candidates[0])
+
+
+def select_numpy_float(candidates: npt.NDArray[np.float64]) -> np.float64:
+    return candidates[0]
+
+
+def select_numpy_float32(candidates: npt.NDArray[np.float64]) -> np.float32:
+    return np.float32(candidates[0])
+
+
+def select_array(candidates: npt.NDArray[np.float64]) -> npt.NDArray[np.float64]:
+    return candidates
+
+
+cashflows = [-5, 10.5, 1, -8, 1]
+default_result: float = npf.irr(cashflows)
+python_result: float = npf.irr(cashflows, selection_logic=select_python_float)
+numpy_result: float = npf.irr(cashflows, selection_logic=select_numpy_float)
+float32_result: float = npf.irr(cashflows, selection_logic=select_numpy_float32)
+npf.irr(cashflows, selection_logic=select_array)  # type: ignore[arg-type]