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..4bfea70 100644
--- a/test_calc.py
+++ b/test_calc.py
@@ -15,6 +15,32 @@ def test_unknown_operator():
         calculate(1, "%", 2)
 
 
+@pytest.mark.parametrize("right", [0, 0.0, -0.0])
+def test_division_by_zero(right):
+    with pytest.raises(CalculatorError, match="division by zero"):
+        calculate(8, "/", right)
+
+
+def test_main_reports_division_by_zero(capsys):
+    assert main(["1", "/", "0"]) == 1
+    captured = capsys.readouterr()
+    assert captured.out == ""
+    assert "division by zero" in captured.err
+    assert "Traceback" not in captured.err
+
+
+def test_main_division_by_zero_exact_message(capsys):
+    assert main(["8", "/", "0.0"]) == 1
+    assert capsys.readouterr().err == "error: division by zero\n"
+
+
+@pytest.mark.parametrize(
+    ("operator", "expected"), [("+", 1), ("-", 1), ("*", 0)]
+)
+def test_zero_right_operand_ok_for_other_operators(operator, expected):
+    assert calculate(1, operator, 0) == expected
+
+
 def test_main_prints_result(capsys):
     assert main(["8", "/", "2"]) == 0
     assert capsys.readouterr().out.strip() == "4.0"