@@ -638,7 +638,7 @@ def split_tf(transfer_function):
                                             array([1, 1]),
                                             name='G', outputs=1, inputs=1)]],
           dtype=object)
-    
+
     """
     tf_split_lst = []
     for i_out in range(transfer_function.noutputs):
 
@@ -64,8 +64,9 @@ def damp(self):
 
     def feedback(self, other=1, sign=-1):
         """Feedback interconnection between two LTI objects."""
-        # Implemented in subclasses, but documented here
-        return NotImplemented
+        raise NotImplementedError(
+            "feedback not implemented for base "
+            f"{self.__class__.__name__} objects")
 
     def frequency_response(self, omega=None, squeeze=None):
         """Evaluate LTI system response at an array of frequencies.
@@ -217,9 +218,6 @@ def ispassive(self):
         # importing here prevents circular dependancy
         from control.passivity import ispassive
         return ispassive(self)
-    
-    def feedback(self, other=1, sign=-1):
-        raise NotImplementedError(f"feedback not implemented for base {self.__class__.__name__} objects")
 
     # convenience aliases
     # most function are only forward declaraed and patched in the __init__.py to avoid circular imports
 
@@ -4,15 +4,16 @@
 
 """
 
-import numpy as np
-from scipy.signal import zpk2tf
 import warnings
 from warnings import warn
 
+import numpy as np
+from scipy.signal import zpk2tf
+
+from ..exception import ControlArgument
+from ..lti import LTI
 from ..statesp import ss
 from ..xferfcn import tf
-from ..lti import LTI
-from ..exception import ControlArgument
 
 __all__ = ['bode', 'nyquist', 'ngrid', 'rlocus', 'pzmap', 'dcgain', 'connect']
 
@@ -127,7 +128,7 @@ def nyquist(*args, plot=True, **kwargs):
         Frequencies in rad/s.
 
     """
-    from ..freqplot import nyquist_response, nyquist_plot
+    from ..freqplot import nyquist_plot, nyquist_response
 
     # If first argument is a list, assume python-control calling format
     if hasattr(args[0], '__iter__'):
@@ -311,6 +312,8 @@ def pzmap(*args, **kwargs):
 
 
 from ..nichols import nichols_grid
+
+
 def ngrid():
     return nichols_grid()
 ngrid.__doc__ = nichols_grid.__doc__
@@ -378,6 +381,8 @@ def dcgain(*args):
 
 
 from ..bdalg import connect as ct_connect
+
+
 def connect(*args):
     """connect(sys, Q, inputv, outputv)
 
 
@@ -20,7 +20,6 @@
 import numpy as np
 import scipy as sp
 import scipy.linalg
-# array needed in eval() call
 from numpy import array  # noqa: F401
 from numpy import any, asarray, concatenate, cos, delete, empty, exp, eye, \
     isinf, pad, sin, squeeze, zeros
@@ -1487,7 +1486,7 @@ def output(self, t, x, u=None, params=None):
                 raise ValueError("len(u) must be equal to number of inputs")
             return (self.C @ x).reshape((-1,)) \
                 + (self.D @ u).reshape((-1,))  # return as row vector
-        
+
     # convenience aliase, import needs to go over the submodule to avoid circular imports
     initial_response = control.timeresp.initial_response