test: div by zero returns nan (#2636)

* Add tests for div by zero

* Use constructor eager for to cover backend testing

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* add lazy div by zero testing

* add collect call to lazy constructors

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Created individual tests for floordiv and truediv by zero

* Add polars backend specific division by zero return value

* Exclude floordiv tests for pandas versions < 2.0

* Added pandas[nullable] conditional for floordiv by zero

* Add float div by zero test for truediv, make new test file

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Ruff fixes

---------

Co-authored-by: Francesco Bruzzesi <[email protected]>

Author: jrw34

tests/expr_and_series/division_by_zero_test.py

@@ -0,0 +1,115 @@
+from __future__ import annotations
+
+import pytest
+
+import narwhals as nw
+from tests.utils import PANDAS_VERSION, Constructor, ConstructorEager, assert_equal_data
+
+
+@pytest.mark.parametrize(
+    ("left", "right", "expected"),
+    [
+        (-2, 0, float("-inf")),
+        (-2.0, 0.0, float("-inf")),
+        (0, 0, None),
+        (0.0, 0.0, None),
+        (2, 0, float("inf")),
+        (2.0, 0.0, float("inf")),
+    ],
+)
+def test_series_truediv_by_zero(
+    left: float, right: float, expected: float | None, constructor_eager: ConstructorEager
+) -> None:
+    data: dict[str, list[int | float]] = {"a": [left], "b": [right]}
+    df = nw.from_native(constructor_eager(data), eager_only=True)
+    truediv_result = df["a"] / df["b"]
+    assert_equal_data({"a": truediv_result}, {"a": [expected]})
+
+
+@pytest.mark.parametrize(
+    ("left", "right", "expected"),
+    [(-2, 0, float("-inf")), (0, 0, None), (2, 0, float("inf"))],
+)
+@pytest.mark.skipif(PANDAS_VERSION < (2, 0), reason="converts floordiv by zero to 0")
+def test_series_floordiv_int_by_zero(
+    left: int,
+    right: int,
+    expected: float | None,
+    constructor_eager: ConstructorEager,
+    request: pytest.FixtureRequest,
+) -> None:
+    data: dict[str, list[int]] = {"a": [left], "b": [right]}
+    df = nw.from_native(constructor_eager(data), eager_only=True)
+    # pyarrow backend floordiv raises divide by zero error
+    if "pyarrow" in str(constructor_eager):
+        request.applymarker(pytest.mark.xfail)
+    # polars backend floordiv by zero always returns null
+    if "polars" in str(constructor_eager):
+        floordiv_result = df["a"] // df["b"]
+        assert all(floordiv_result.is_null())
+    # pandas[nullable] backend floordiv always returns 0
+    elif all(x in str(constructor_eager) for x in ["pandas", "nullable"]):
+        floordiv_result = df["a"] // df["b"]
+        assert_equal_data({"a": floordiv_result}, {"a": [0]})
+    else:
+        floordiv_result = df["a"] // df["b"]
+        assert_equal_data({"a": floordiv_result}, {"a": [expected]})
+
+
+@pytest.mark.parametrize(
+    ("left", "right", "expected"),
+    [
+        (-2, 0, float("-inf")),
+        (-2.0, 0.0, float("-inf")),
+        (0, 0, None),
+        (0.0, 0.0, None),
+        (2, 0, float("inf")),
+        (2.0, 0.0, float("inf")),
+    ],
+)
+def test_truediv_by_zero(
+    left: float, right: float, expected: float | None, constructor: Constructor
+) -> None:
+    data: dict[str, list[int | float]] = {"a": [left]}
+    df = nw.from_native(constructor(data))
+    truediv_result = df.select(nw.col("a") / right)
+    assert_equal_data(truediv_result, {"a": [expected]})
+
+
+@pytest.mark.parametrize(
+    ("left", "right", "expected"),
+    [(-2, 0, float("-inf")), (0, 0, None), (2, 0, float("inf"))],
+)
+@pytest.mark.skipif(PANDAS_VERSION < (2, 0), reason="converts floordiv by zero to 0")
+def test_floordiv_int_by_zero(
+    left: int,
+    right: int,
+    expected: float | None,
+    constructor: Constructor,
+    request: pytest.FixtureRequest,
+) -> None:
+    data: dict[str, list[int]] = {"a": [left]}
+    df = nw.from_native(constructor(data))
+    # pyarrow backend floordiv raises divide by zero error
+    # ibis backend floordiv cannot cast value to inf or -inf
+    if any(x in str(constructor) for x in ["ibis", "pyarrow"]):
+        request.applymarker(pytest.mark.xfail)
+    # duckdb backend floordiv return None
+    if "duckdb" in str(constructor):
+        floordiv_result = df.select(nw.col("a") // right)
+        assert_equal_data(floordiv_result, {"a": [None]})
+    # polars backend floordiv returns null
+    elif "polars" in str(constructor) and "lazy" not in str(constructor):
+        floordiv_result = df.select(nw.col("a") // right)
+        assert all(floordiv_result["a"].is_null())
+    # polars lazy floordiv cannot be sliced and returns None
+    elif all(x in str(constructor) for x in ["polars", "lazy"]):
+        floordiv_result = df.select(nw.col("a") // right)
+        assert_equal_data(floordiv_result, {"a": [None]})
+    # pandas[nullable] backend floordiv always returns 0
+    elif all(x in str(constructor) for x in ["pandas", "nullable"]):
+        floordiv_result = df.select(nw.col("a") // right)
+        assert_equal_data(floordiv_result, {"a": [0]})
+    else:
+        floordiv_result = df.select(nw.col("a") // right)
+        assert_equal_data(floordiv_result, {"a": [expected]})