@@ -0,0 +1,29 @@
+# run ruff check on library source
+# TODO: extend to tests, examples, benchmarks
+
+name: ruff-check
+
+on: [push, pull_request]
+
+jobs:
+  ruff-check-linux:
+    # ruff *shouldn't* be sensitive to platform
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout python-control
+      uses: actions/checkout@v3
+
+    - name: Setup environment
+      uses: actions/setup-python@v4
+      with:
+        python-version: 3.13 # todo: latest?
+
+    - name: Install ruff
+      run: |
+        python -m pip install --upgrade pip
+        python -m pip install ruff
+
+    - name: Run ruff check
+      run: |
+        ruff check
@@ -70,6 +70,11 @@
 # Import functions from within the control system library
 # Note: the functions we use are specified as __all__ variables in the modules
 
+# don't warn about `import *`
+# ruff: noqa: F403
+# don't warn about unknown names; they come via `import *`
+# ruff: noqa: F405
+
 # Input/output system modules
 from .iosys import *
 from .nlsys import *
@@ -106,8 +111,8 @@
 from .sysnorm import *
 
 # Allow access to phase_plane functions as ct.phaseplot.fcn or ct.pp.fcn
-from . import phaseplot
-from . import phaseplot as pp
+from . import phaseplot as phaseplot
+pp = phaseplot
 
 # Exceptions
 from .exception import *
 
@@ -8,9 +8,8 @@
 
 import numpy as np
 
-from numpy import zeros, zeros_like, shape, poly, iscomplex, vstack, hstack, \
-    transpose, empty, finfo, float64
-from numpy.linalg import solve, matrix_rank, eig
+from numpy import zeros_like, poly, transpose
+from numpy.linalg import solve, matrix_rank
 
 from scipy.linalg import schur
 
 
@@ -48,7 +48,6 @@
 """
 
 from .iosys import isctime
-from .statesp import StateSpace
 
 __all__ = ['sample_system', 'c2d']
 
 
@@ -71,7 +71,7 @@ def slycot_check():
     global slycot_installed
     if slycot_installed is None:
         try:
-            import slycot
+            import slycot # noqa: F401
             slycot_installed = True
         except:
             slycot_installed = False
@@ -85,7 +85,7 @@ def pandas_check():
     global pandas_installed
     if pandas_installed is None:
         try:
-            import pandas
+            import pandas # noqa: F401
             pandas_installed = True
         except:
             pandas_installed = False
@@ -98,7 +98,7 @@ def cvxopt_check():
     global cvxopt_installed
     if cvxopt_installed is None:
         try:
-            import cvxopt
+            import cvxopt # noqa: F401
             cvxopt_installed = True
         except:
             cvxopt_installed = False
 
@@ -61,15 +61,17 @@
 """
 
 # Basis function families
-from .basis import BasisFamily
-from .poly import PolyFamily
-from .bezier import BezierFamily
-from .bspline import BSplineFamily
+from .basis import BasisFamily as BasisFamily
+from .poly import PolyFamily as PolyFamily
+from .bezier import BezierFamily as BezierFamily
+from .bspline import BSplineFamily as BSplineFamily
 
 # Classes
-from .systraj import SystemTrajectory
-from .flatsys import FlatSystem, flatsys
-from .linflat import LinearFlatSystem
+from .systraj import SystemTrajectory as SystemTrajectory
+from .flatsys import FlatSystem as FlatSystem
+from .flatsys import flatsys as flatsys
+from .linflat import LinearFlatSystem as LinearFlatSystem
 
 # Package functions
-from .flatsys import point_to_point, solve_flat_ocp
+from .flatsys import point_to_point as point_to_point
+from .flatsys import solve_flat_ocp as solve_flat_ocp
@@ -8,7 +8,7 @@
 
 import numpy as np
 from .basis import BasisFamily
-from scipy.interpolate import BSpline, splev
+from scipy.interpolate import BSpline
 
 class BSplineFamily(BasisFamily):
     """B-spline basis functions.
 
@@ -8,6 +8,7 @@
 import warnings
 from .poly import PolyFamily
 from .systraj import SystemTrajectory
+from ..exception import ControlArgument
 from ..nlsys import NonlinearIOSystem
 from ..timeresp import _check_convert_array
 
@@ -245,7 +246,6 @@ def flatsys(*args, updfcn=None, outfcn=None, **kwargs):
     """
     from .linflat import LinearFlatSystem
     from ..statesp import StateSpace
-    from ..iosys import _process_iosys_keywords
 
     if len(args) == 1 and isinstance(args[0], StateSpace):
         # We were passed a linear system, so call linflat
 
@@ -15,8 +15,8 @@
 from warnings import warn
 
 import numpy as np
-from numpy import absolute, angle, array, empty, eye, imag, linalg, ones, \
-    real, sort, where
+from numpy import absolute, array, empty, eye, imag, linalg, ones, \
+    real, sort
 from scipy.interpolate import splev, splprep
 
 from . import config
 
@@ -11,7 +11,6 @@
 import itertools
 import math
 import warnings
-from os.path import commonprefix
 
 import matplotlib as mpl
 import matplotlib.pyplot as plt
@@ -760,13 +759,11 @@ def _make_line_label(response, output_index, input_index):
             if plot_magnitude:
                 ax_mag.axhline(y=0 if dB else 1, color='k', linestyle=':',
                                zorder=-20)
-                mag_ylim = ax_mag.get_ylim()
 
             if plot_phase:
                 ax_phase.axhline(y=phase_limit if deg else
                                  math.radians(phase_limit),
                                  color='k', linestyle=':', zorder=-20)
-                phase_ylim = ax_phase.get_ylim()
 
             # Annotate the phase margin (if it exists)
             if plot_phase and pm != float('inf') and Wcp != float('nan'):
@@ -2222,7 +2219,7 @@ def gangof4_plot(
         See :class:`ControlPlot` for more detailed information.
 
     """
-    if len(args) == 1 and isinstance(arg, FrequencyResponseData):
+    if len(args) == 1 and isinstance(args[0], FrequencyResponseData):
         if any([kw is not None
                 for kw in [omega, omega_limits, omega_num, Hz]]):
             raise ValueError(