TST: add branch coverage for fin_flutter_analysis and fix keyword argument bug (#904)

* TST: add branch coverage for fin_flutter_analysis and fix keyword bug

This commit increases test coverage for utilities.py by adding three new
tests that cover previously untested branches in the fin_flutter_analysis
function. The tests cover the see_prints=True branch, see_graphs=True
branch, and both flags enabled simultaneously.

During testing, discovered and fixed a bug where the filename parameter
was incorrectly passed as a positional argument to _flutter_plots, which
requires it as a keyword-only argument. This would cause a TypeError at
runtime when users attempted to save flutter plots to files.

Changes:
- Add test_fin_flutter_analysis_with_prints for print branch coverage
- Add test_fin_flutter_analysis_with_graphs for plotting branch coverage
- Add test_fin_flutter_analysis_complete_output for combined coverage
- Fix filename argument bug in fin_flutter_analysis (line 286)
- Fix test_flutter_plots to use keyword argument for filename
- Increases utilities.py coverage from 68% to 69% (+2 statements)

* DOC: updated CHANGELOG for test coverage and bug fix

* DOC: revert CHANGELOG.md changes per reviewer feedback

Author: erielC

rocketpy/utilities.py

@@ -283,7 +283,7 @@ def fin_flutter_analysis(
             flight,
         )
     if see_graphs:
-        _flutter_plots(flight, flutter_mach, safety_factor, filename)
+        _flutter_plots(flight, flutter_mach, safety_factor, filename=filename)
     else:
         return flutter_mach, safety_factor
 

tests/unit/test_utilities.py

@@ -117,6 +117,76 @@ def test_fin_flutter_analysis(flight_calisto_custom_wind):
     assert np.isclose(safety_factor(np.inf), 61.669562809629035, atol=5e-3)
 
 
+def test_fin_flutter_analysis_with_prints(flight_calisto_custom_wind):
+    """Test fin_flutter_analysis with see_prints=True to cover print branch.
+
+    Parameters
+    ----------
+    flight_calisto_custom_wind : Flight
+        A Flight object with a rocket with fins.
+    """
+    flutter_mach, safety_factor = utilities.fin_flutter_analysis(
+        fin_thickness=2 / 1000,
+        shear_modulus=10e9,
+        flight=flight_calisto_custom_wind,
+        see_prints=True,
+        see_graphs=False,  # False = returns tuple!
+        filename=None,
+    )
+
+    # Verify returns are valid
+    assert flutter_mach is not None
+    assert safety_factor is not None
+
+
+@patch("matplotlib.pyplot.show")
+def test_fin_flutter_analysis_with_graphs(mock_show, flight_calisto_custom_wind):  # pylint: disable=unused-argument
+    """Test fin_flutter_analysis with see_graphs=True to cover plotting branch.
+
+    Parameters
+    ----------
+    mock_show : mock
+        Mock of matplotlib.pyplot.show function.
+    flight_calisto_custom_wind : Flight
+        A Flight object with a rocket with fins.
+    """
+    result = utilities.fin_flutter_analysis(
+        fin_thickness=2 / 1000,
+        shear_modulus=10e9,
+        flight=flight_calisto_custom_wind,
+        see_prints=False,
+        see_graphs=True,  # True = returns None!
+        filename=None,
+    )
+
+    assert result is None
+    mock_show.assert_called()
+
+
+@patch("matplotlib.pyplot.show")
+def test_fin_flutter_analysis_complete_output(mock_show, flight_calisto_custom_wind):  # pylint: disable=unused-argument
+    """Test fin_flutter_analysis with both prints and graphs enabled.
+
+    Parameters
+    ----------
+    mock_show : mock
+        Mock of matplotlib.pyplot.show function.
+    flight_calisto_custom_wind : Flight
+        A Flight object with a rocket with fins.
+    """
+    result = utilities.fin_flutter_analysis(
+        fin_thickness=2 / 1000,
+        shear_modulus=10e9,
+        flight=flight_calisto_custom_wind,
+        see_prints=True,
+        see_graphs=True,  # True = returns None!
+        filename=None,
+    )
+
+    assert result is None
+    mock_show.assert_called()
+
+
 def test_flutter_prints(flight_calisto_custom_wind):
     """Tests the _flutter_prints function.