Reject nonpositive interval range amounts

docs/docs/interval.md

@@ -138,7 +138,9 @@ If you want to iterate over a interval, you can use the `range()` method:
     Supported units for `range()` are: `years`, `months`, `weeks`,
     `days`, `hours`, `minutes`, `seconds` and `microseconds`
 
-You can pass an amount for the passed unit to control the length of the gap:
+You can pass a positive amount for the passed unit to control the length of the gap.
+Zero or negative amounts raise `ValueError` when iteration starts. Inverted intervals
+already iterate backwards, so their amount must also be positive:
 
 ```python
 >>> for dt in interval.range('days', 2):

src/pendulum/interval.py

@@ -288,6 +288,9 @@ def in_words(self, locale: str | None = None, separator: str = " ") -> str:
         return separator.join(parts)
 
     def range(self, unit: str, amount: int = 1) -> Iterator[_T]:
+        if amount <= 0:
+            raise ValueError("amount must be positive")
+
         method = "add"
         op = operator.le
         if not self._absolute and self.invert:

tests/interval/test_range_amount_regression.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+import pendulum
+
+
+if TYPE_CHECKING:
+    from collections.abc import Callable
+
+
+@pytest.mark.parametrize("factory", [pendulum.date, pendulum.datetime])
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize("absolute", [False, True])
+@pytest.mark.parametrize("amount", [0, -1, -2])
+def test_range_rejects_nonpositive_amount(
+    factory: Callable[[int, int, int], pendulum.Date],
+    reverse: bool,
+    absolute: bool,
+    amount: int,
+) -> None:
+    start = factory(2024, 1, 10)
+    end = factory(2024, 1, 14)
+    if reverse:
+        start, end = end, start
+    interval = pendulum.Interval(start, end, absolute=absolute)
+    with pytest.raises(ValueError, match="amount must be positive"):
+        next(interval.range("days", amount))
+
+
+@pytest.mark.parametrize("amount", [0, -1])
+def test_single_point_range_rejects_nonpositive_amount(amount: int) -> None:
+    point = pendulum.date(2024, 1, 10)
+    interval = pendulum.Interval(point, point)
+    with pytest.raises(ValueError, match="amount must be positive"):
+        next(interval.range("days", amount))
+
+
+@pytest.mark.parametrize("factory", [pendulum.date, pendulum.datetime])
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize("amount", [1, 2, 3, 5])
+def test_positive_range_amount_uses_interval_direction(
+    factory: Callable[[int, int, int], pendulum.Date], reverse: bool, amount: int
+) -> None:
+    start = factory(2024, 1, 10)
+    end = factory(2024, 1, 14)
+    if reverse:
+        start, end = end, start
+    interval = pendulum.Interval(start, end)
+    direction = -1 if reverse else 1
+    expected = [
+        factory(2024, 1, day)
+        for day in range(start.day, end.day + direction, direction * amount)
+    ]
+    assert list(interval.range("days", amount)) == expected