test for loop_from_index

src/textual/_loop.py

@@ -46,11 +46,17 @@ def loop_first_last(values: Iterable[T]) -> Iterable[tuple[bool, bool, T]]:
 
 
 def loop_from_index(
-    values: Sequence[T], index: int, direction: Literal[+1, -1] = +1, wrap: bool = True
+    values: Sequence[T],
+    index: int,
+    direction: Literal[-1, +1] = +1,
+    wrap: bool = True,
 ) -> Iterable[tuple[int, T]]:
     """Iterate over values in a sequence from a given starting index, potentially wrapping the index
     if it would go out of bounds.
 
+    Note that the first value to be yielded is a step from `index`, and `index` will be yielded *last*.
+
+
     Args:
         values: A sequence of values.
         index: Starting index.
@@ -60,6 +66,8 @@ def loop_from_index(
     Yields:
         A tuple of index and value from the sequence.
     """
+    # Sanity check for devs who miss the typing errors
+    assert direction in (-1, +1), "direction must be -1 or +1"
     count = len(values)
     if wrap:
         for _ in range(count):

tests/test_loop.py

@@ -1,4 +1,4 @@
-from textual._loop import loop_first, loop_first_last, loop_last
+from textual._loop import loop_first, loop_first_last, loop_from_index, loop_last
 
 
 def test_loop_first():
@@ -26,3 +26,40 @@ def test_loop_first_last():
     assert next(iterable) == (False, False, "oranges")
     assert next(iterable) == (False, False, "pears")
     assert next(iterable) == (False, True, "lemons")
+
+
+def test_loop_from_index():
+    assert list(loop_from_index("abcdefghij", 3)) == [
+        (4, "e"),
+        (5, "f"),
+        (6, "g"),
+        (7, "h"),
+        (8, "i"),
+        (9, "j"),
+        (0, "a"),
+        (1, "b"),
+        (2, "c"),
+        (3, "d"),
+    ]
+
+    assert list(loop_from_index("abcdefghij", 3, direction=-1)) == [
+        (2, "c"),
+        (1, "b"),
+        (0, "a"),
+        (9, "j"),
+        (8, "i"),
+        (7, "h"),
+        (6, "g"),
+        (5, "f"),
+        (4, "e"),
+        (3, "d"),
+    ]
+
+    assert list(loop_from_index("abcdefghij", 3, wrap=False)) == [
+        (4, "e"),
+        (5, "f"),
+        (6, "g"),
+        (7, "h"),
+        (8, "i"),
+        (9, "j"),
+    ]