Fix Interval.range hang on a zero or wrong-sign amount

range() treated amount as both the step and the termination
increment with no validation, so amount=0 yielded the start
forever and a mismatched sign walked away from the end.

Reject a zero amount with ValueError. A negative amount whose
sign does not match the interval direction now yields an empty
iterator, matching Python range().

Author: ekanshul

docs/docs/interval.md

@@ -151,6 +151,10 @@ You can pass an amount for the passed unit to control the length of the gap:
 '2000-01-09T00:00:00+00:00'
 ```
 
+`amount` must be a non-zero integer. A zero amount raises `ValueError`.
+A negative amount whose sign does not match the interval direction
+yields an empty iterator (the same way Python's `range()` does).
+
 You can also directly iterate over the `Interval` instance,
 the unit will be `days` in this case:
 

src/pendulum/interval.py

@@ -288,12 +288,23 @@ 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("range() requires a non-zero amount")
+
         method = "add"
         op = operator.le
         if not self._absolute and self.invert:
             method = "subtract"
             op = operator.ge
 
+        # A negative amount walks the opposite direction of the interval.
+        # Flip the step so the loop either iterates toward the end (matching
+        # sign) or is empty immediately (wrong sign), instead of hanging.
+        if amount < 0:
+            amount = -amount
+            method = "subtract" if method == "add" else "add"
+            op = operator.ge if op is operator.le else operator.le
+
         start, end = self.start, self.end
 
         i = amount

tests/interval/test_range.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import pytest
+
 import pendulum
 
 from pendulum.interval import Interval
@@ -117,3 +119,23 @@ def test_range_amount():
     assert_datetime(r[1], 2016, 10, 16, 1, 0, 0)
     assert_datetime(r[2], 2016, 10, 18, 0, 0, 0)
     assert_datetime(r[3], 2016, 10, 20, 0, 0, 0)
+
+
+def test_range_zero_amount_raises():
+    dt1 = pendulum.datetime(2000, 1, 1)
+    dt2 = pendulum.datetime(2000, 1, 10)
+    p = pendulum.interval(dt1, dt2)
+
+    with pytest.raises(ValueError, match="non-zero amount"):
+        list(p.range("days", 0))
+
+
+def test_range_wrong_sign_amount_is_empty():
+    dt1 = pendulum.datetime(2000, 1, 1)
+    dt2 = pendulum.datetime(2000, 1, 10)
+
+    forward = pendulum.interval(dt1, dt2)
+    assert list(forward.range("days", -1)) == []
+
+    inverted = pendulum.interval(dt2, dt1)
+    assert list(inverted.range("days", -1)) == []