TST: add a test demonstrating perturbation confusion handling. (#688)

Co-authored-by: JHM Darbyshire (win11) <[email protected]>

pyproject.toml

@@ -43,7 +43,7 @@ dev = [
     "mypy>=1.13,<2.0",
     "pandas-stubs>2.0,<3.0",
     # doc building
-    "sphinx>=8.0,<9.0",
+    "sphinx>=8.0,<8.2",
     "sphinx-automodapi>=0.16.0,<1.0",
     "sphinxcontrib-googleanalytics>=0.4,<1.0",
     "sphinx-tabs>=3.4,<4.0",

python/tests/test_dual.py

@@ -712,6 +712,22 @@ def test_variable_set_order() -> None:
     assert isinstance(x_dual2, Dual2)
 
 
+def test_perturbation_confusion() -> None:
+    # https://www.bcl.hamilton.ie/~barak/papers/ifl2005.pdf
+
+    # Utilised tagged variables
+    x = Dual(1.0, ["x"], [])
+    y = Dual(1.0, ["y"], [])
+    result = gradient(x * gradient(x + y, ["y"]), ["x"])
+    assert result == 1.0
+
+    # Replicates untagged variables
+    x = Dual(1.0, ["x"], [])
+    y = Dual(1.0, ["x"], [])
+    result = gradient(x * gradient(x + y, ["x"]), ["x"])
+    assert result == 2.0
+
+
 # Linalg dual_solve tests
 
 

requirements.txt

@@ -9,7 +9,7 @@ pytest>=8.3.2,<9.0
 coverage>=7.6.1,<8.0  # for test and code monitoring
 
 # doc building
-sphinx>=8.0,<9.0
+sphinx>=8.0,<8.2
 sphinx-automodapi>=0.16.0,<1.0
 sphinxcontrib-googleanalytics>=0.4,<1.0
 sphinx-tabs>=3.4,<4.0