Add expression-evaluator: DAGs & state machines tutorial project

Educational calculator teaching FSMs (explicit transition table tokenizer)
and DAGs (recursive descent parser with AST evaluation). Includes CLI with
REPL, graphviz visualization, and 61 tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/expression-evaluator/tests/test_evaluator.py

@@ -0,0 +1,120 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+import pytest
+from tokenizer import tokenize
+from parser import Parser
+from evaluator import evaluate, evaluate_traced, EvalError
+
+
+def eval_expr(expr):
+    """Helper: tokenize -> parse -> evaluate in one step."""
+    tokens = tokenize(expr)
+    ast = Parser(tokens).parse()
+    return evaluate(ast)
+
+
+# ---------- Basic arithmetic ----------
+
+def test_addition():
+    assert eval_expr("3 + 4") == 7.0
+
+def test_subtraction():
+    assert eval_expr("10 - 3") == 7.0
+
+def test_multiplication():
+    assert eval_expr("3 * 4") == 12.0
+
+def test_division():
+    assert eval_expr("10 / 4") == 2.5
+
+def test_power():
+    assert eval_expr("2 ^ 10") == 1024.0
+
+
+# ---------- Precedence ----------
+
+def test_standard_precedence():
+    assert eval_expr("3 + 4 * 2") == 11.0
+
+def test_parentheses():
+    assert eval_expr("(3 + 4) * 2") == 14.0
+
+def test_power_precedence():
+    assert eval_expr("2 * 3 ^ 2") == 18.0
+
+def test_right_associative_power():
+    # 2^(2^3) = 2^8 = 256
+    assert eval_expr("2 ^ 2 ^ 3") == 256.0
+
+
+# ---------- Unary minus ----------
+
+def test_negation():
+    assert eval_expr("-5") == -5.0
+
+def test_double_negation():
+    assert eval_expr("--5") == 5.0
+
+def test_negation_with_power():
+    # -(3^2) = -9, not (-3)^2 = 9
+    assert eval_expr("-3 ^ 2") == -9.0
+
+def test_negation_in_parens():
+    assert eval_expr("(-3) ^ 2") == 9.0
+
+
+# ---------- Decimals ----------
+
+def test_decimal_addition():
+    assert eval_expr("0.1 + 0.2") == pytest.approx(0.3)
+
+def test_leading_dot():
+    assert eval_expr(".5 + .5") == 1.0
+
+
+# ---------- Edge cases ----------
+
+def test_nested_parens():
+    assert eval_expr("((((3))))") == 3.0
+
+def test_complex_expression():
+    assert eval_expr("(2 + 3) * (7 - 2) / 5 ^ 1") == 5.0
+
+def test_long_chain():
+    assert eval_expr("1 + 2 + 3 + 4 + 5") == 15.0
+
+def test_mixed_operations():
+    assert eval_expr("2 + 3 * 4 - 6 / 2") == 11.0
+
+
+# ---------- Division by zero ----------
+
+def test_division_by_zero():
+    with pytest.raises(EvalError):
+        eval_expr("1 / 0")
+
+def test_division_by_zero_in_expression():
+    with pytest.raises(EvalError):
+        eval_expr("5 + 3 / (2 - 2)")
+
+
+# ---------- Traced evaluation ----------
+
+def test_traced_returns_correct_result():
+    tokens = tokenize("3 + 4 * 2")
+    ast = Parser(tokens).parse()
+    result, steps = evaluate_traced(ast)
+    assert result == 11.0
+    assert len(steps) > 0
+
+def test_traced_step_count():
+    """A simple binary op has 3 evaluation events: left, right, combine."""
+    tokens = tokenize("3 + 4")
+    ast = Parser(tokens).parse()
+    result, steps = evaluate_traced(ast)
+    assert result == 7.0
+    # NumberNode(3), NumberNode(4), BinOp(+)
+    assert len(steps) == 3