Division by zero crashes

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Final diff

diff --git a/calc.py b/calc.py
index f16c6ca..b7d522d 100644
--- a/calc.py
+++ b/calc.py
@@ -15,6 +15,8 @@ def calculate(left: float, operator: str, right: float) -> float:
     if operator == "*":
         return left * right
     if operator == "/":
+        if right == 0:
+            raise CalculatorError("division by zero")
         return left / right
     raise CalculatorError(f"unknown operator: {operator}")
 
diff --git a/test_calc.py b/test_calc.py
index 856b0c9..e1a274e 100644
--- a/test_calc.py
+++ b/test_calc.py
@@ -23,3 +23,35 @@ def test_main_prints_result(capsys):
 def test_main_reports_unknown_operator(capsys):
     assert main(["1", "%", "2"]) == 1
     assert "unknown operator" in capsys.readouterr().err
+
+
+@pytest.mark.parametrize("right", [0, 0.0, -0.0])
+def test_division_by_zero_raises(right):
+    with pytest.raises(CalculatorError, match="division by zero"):
+        calculate(1, "/", right)
+
+
+def test_main_reports_division_by_zero(capsys):
+    assert main(["1", "/", "0"]) == 1
+    captured = capsys.readouterr()
+    assert "division by zero" in captured.err
+    assert "Traceback" not in captured.err
+    assert captured.out == ""
+
+
+def test_cli_division_by_zero_exits_with_status_1():
+    import subprocess
+    import sys
+    from pathlib import Path
+
+    script = Path(__file__).parent / "calc.py"
+    proc = subprocess.run(
+        [sys.executable, str(script), "1", "/", "0"],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert proc.returncode == 1
+    assert "division by zero" in proc.stderr
+    assert "Traceback" not in proc.stderr
+    assert proc.stdout == ""