test(examples): add pytest tests for mathkit Calculator (ABCA-733)

examples/mathkit/test_calculator.py

@@ -0,0 +1,88 @@
+"""Pytest tests for the mathkit :class:`Calculator`.
+
+These tests import :class:`Calculator` from :mod:`calculator` (the sibling
+module in this package) and exercise its arithmetic methods, method
+chaining, reset behaviour, and divide-by-zero error handling.
+
+Run from within ``examples/mathkit`` so the sibling ``calculator`` /
+``operations`` modules are importable, e.g.::
+
+    cd examples/mathkit && pytest
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from calculator import Calculator
+
+
+def test_initial_value_defaults_to_zero() -> None:
+    assert Calculator().value == 0.0
+
+
+def test_initial_value_is_configurable() -> None:
+    assert Calculator(10).value == 10
+
+
+def test_add() -> None:
+    assert Calculator(2).add(3).value == 5
+
+
+def test_subtract() -> None:
+    assert Calculator(5).subtract(2).value == 3
+
+
+def test_multiply() -> None:
+    assert Calculator(4).multiply(3).value == 12
+
+
+def test_divide() -> None:
+    assert Calculator(9).divide(3).value == 3
+
+
+def test_divide_by_zero_raises_value_error() -> None:
+    with pytest.raises(ValueError):
+        Calculator(1).divide(0)
+
+
+def test_reset_defaults_to_zero() -> None:
+    calc = Calculator(42)
+    calc.reset()
+    assert calc.value == 0.0
+
+
+def test_reset_to_specific_value() -> None:
+    calc = Calculator(42)
+    calc.reset(7)
+    assert calc.value == 7
+
+
+def test_methods_return_self_for_chaining() -> None:
+    calc = Calculator(1)
+    assert calc.add(1) is calc
+    assert calc.subtract(1) is calc
+    assert calc.multiply(2) is calc
+    assert calc.divide(2) is calc
+    assert calc.reset() is calc
+
+
+def test_method_chaining_matches_docstring_example() -> None:
+    assert Calculator(2).add(3).multiply(4).value == 20.0
+
+
+def test_longer_chain() -> None:
+    result = Calculator(10).add(5).subtract(3).multiply(2).divide(4).value
+    assert result == 6.0
+
+
+@pytest.mark.parametrize(
+    ("start", "delta", "expected"),
+    [
+        (0, 0, 0),
+        (-5, 5, 0),
+        (2.5, 0.5, 3.0),
+    ],
+)
+def test_add_parametrized(start: float, delta: float, expected: float) -> None:
+    assert Calculator(start).add(delta).value == expected