diff --git a/src/power_grid_model_ds/_core/fancypy.py b/src/power_grid_model_ds/_core/fancypy.py
index ef4dc05..793618b 100644
--- a/src/power_grid_model_ds/_core/fancypy.py
+++ b/src/power_grid_model_ds/_core/fancypy.py
@@ -4,17 +4,19 @@
 
 """A set of helper functions that mimic numpy functions but are specifically designed for FancyArrays."""
 
-from typing import TYPE_CHECKING, Union
+from typing import TYPE_CHECKING, TypeVar, Union
 
 import numpy as np
 
 from power_grid_model_ds._core.utils.misc import array_equal_with_nan
 
 if TYPE_CHECKING:
-    from power_grid_model_ds._core.model.arrays.base.array import FancyArray
+    from power_grid_model_ds._core.model.arrays.base.array import FancyArray  # noqa
 
+T = TypeVar("T", bound="FancyArray")
 
-def concatenate(fancy_array: "FancyArray", *other_arrays: Union["FancyArray", np.ndarray]) -> "FancyArray":
+
+def concatenate(fancy_array: T, *other_arrays: Union[T, np.ndarray]) -> T:
     """Concatenate arrays."""
     np_arrays = [array if isinstance(array, np.ndarray) else array.data for array in other_arrays]
     try:
@@ -24,7 +26,7 @@ def concatenate(fancy_array: "FancyArray", *other_arrays: Union["FancyArray", np
     return fancy_array.__class__(data=concatenated)
 
 
-def unique(array: "FancyArray", **kwargs):
+def unique(array: T, **kwargs):
     """Return the unique elements of the array."""
     for column in array.columns:
         if np.issubdtype(array.dtype[column], np.floating) and np.isnan(array[column]).any():
@@ -37,13 +39,13 @@ def unique(array: "FancyArray", **kwargs):
     return array.__class__(data=unique_data)
 
 
-def sort(array: "FancyArray", axis=-1, kind=None, order=None) -> "FancyArray":
+def sort(array: T, axis=-1, kind=None, order=None) -> T:
     """Sort the array in-place and return sorted array."""
     array.data.sort(axis=axis, kind=kind, order=order)
     return array
 
 
-def array_equal(array1: "FancyArray", array2: "FancyArray", equal_nan: bool = True) -> bool:
+def array_equal(array1: T, array2: T, equal_nan: bool = True) -> bool:
     """Return True if two arrays are equal."""
     if equal_nan:
         return array_equal_with_nan(array1.data, array2.data)