Update islice to align behavior closer to CP

adafruit_itertools/__init__.py

@@ -342,17 +342,34 @@ def islice(p, start, stop=(), step=1):
     if stop == ():
         stop = start
         start = 0
+    if stop is not None and stop < 0:
+        raise ValueError("Stop for islice must be None or a non-negative integer")
+    if start < 0:
+        raise ValueError("Start for islice must be a non-negative integer")
+    if step <= 0:
+        raise ValueError("Step for islice must be a positive integer")
+
     # TODO: optimizing or breaking semantics?
     if stop is not None and start >= stop:
         return
     it = iter(p)
     for _ in range(start):
-        next(it)
+        try:
+            next(it)
+        except StopIteration:
+            return
 
     while True:
-        yield next(it)
+        try:
+            val = next(it)
+        except StopIteration:
+            return
+        yield val
         for _ in range(step - 1):
-            next(it)
+            try:
+                next(it)
+            except StopIteration:
+                return
         start += step
         if stop is not None and start >= stop:
             return

tests/README.rst

@@ -0,0 +1,24 @@
+..
+  SPDX-FileCopyrightText: KB Sriram
+  SPDX-License-Identifier: MIT
+..
+
+Itertools Tests
+===============
+
+These tests run under CPython, and are intended to verify that the
+Adafruit library functions return the same outputs compared to ones in
+the standard `itertools` module.
+
+These tests run automatically from the standard `circuitpython github
+workflow <wf_>`_. To run them manually, first install these packages
+if necessary::
+
+  $ pip3 install pytest
+
+Then ensure you're in the *root* directory of the repository and run
+the following command::
+
+  $ python -m pytest
+
+.. _wf: https://github.com/adafruit/workflows-circuitpython-libs/blob/6e1562eaabced4db1bd91173b698b1cc1dfd35ab/build/action.yml#L78-L84

tests/test_itertools.py

@@ -0,0 +1,75 @@
+# SPDX-FileCopyrightText: KB Sriram
+# SPDX-License-Identifier: MIT
+
+from typing import Iterator, Optional, Sequence, TypeVar
+import itertools as it
+import pytest
+import adafruit_itertools as ait
+
+_T = TypeVar("_T")
+
+
+@pytest.mark.parametrize(
+    "seq, start",
+    [
+        ("", 0),
+        ("", 2),
+        ("ABCDEFG", 0),
+        ("ABCDEFG", 2),
+        ("ABCDEFG", 20),
+    ],
+)
+def test_islice_start(seq: Sequence[_T], start: int) -> None:
+    x: Iterator[_T] = ait.islice(seq, start)
+    y: Iterator[_T] = it.islice(seq, start)
+    assert list(x) == list(y)
+
+
+@pytest.mark.parametrize(
+    "seq, start, stop",
+    [
+        ("", 0, 5),
+        ("", 2, 5),
+        ("", 0, 0),
+        ("ABCDEFG", 2, 2),
+        ("ABCDEFG", 2, 6),
+        ("ABCDEFG", 2, None),
+        ("ABCDEFG", 2, 17),
+        ("ABCDEFG", 20, 30),
+    ],
+)
+def test_islice_start_stop(seq: Sequence[_T], start: int, stop: Optional[int]) -> None:
+    x: Iterator[_T] = ait.islice(seq, start, stop)
+    y: Iterator[_T] = it.islice(seq, start, stop)
+    assert list(x) == list(y)
+
+
+@pytest.mark.parametrize(
+    "seq, start, stop, step",
+    [
+        ("", 0, 5, 3),
+        ("", 2, 5, 2),
+        ("", 0, 0, 1),
+        ("ABCDEFG", 2, 2, 2),
+        ("ABCDEFG", 2, 6, 3),
+        ("ABCDEFG", 2, 17, 2),
+        ("ABCDEFG", 0, None, 2),
+        ("ABCDEFG", 20, 30, 3),
+        ("ABCDEFG", 0, None, 3),
+    ],
+)
+def test_islice_start_stop_step(
+    seq: Sequence[_T], start: int, stop: Optional[int], step: int
+) -> None:
+    x: Iterator[_T] = ait.islice(seq, start, stop, step)
+    y: Iterator[_T] = it.islice(seq, start, stop, step)
+    assert list(x) == list(y)
+
+
+def test_islice_error() -> None:
+    with pytest.raises(ValueError):
+        list(ait.islice("abc", -1))
+    with pytest.raises(ValueError):
+        list(ait.islice("abc", 0, -1))
+    with pytest.raises(ValueError):
+        list(ait.islice("abc", 0, 0, 0))