Merge 7c53a74 into b5700b4

more_itertools/more.py

@@ -2121,9 +2121,6 @@ class seekable:
         >>> next(it), next(it), next(it)
         ('0', '1', '2')
 
-    The cache grows as the source iterable progresses, so beware of wrapping
-    very large or infinite iterables.
-
     You may view the contents of the cache with the :meth:`elements` method.
     That returns a :class:`SequenceView`, a view that updates automatically:
 
@@ -2138,11 +2135,30 @@ class seekable:
         >>> elements
         SequenceView(['0', '1', '2', '3'])
 
+    By default, the cache grows as the source iterable progresses, so beware of
+    wrapping very large or infinite iterables. Supply *maxlen* to limit the
+    size of the cache (this of course limits how far back you can seek).
+
+        >>> from itertools import count
+        >>> it = seekable((str(n) for n in count()), maxlen=2)
+        >>> next(it), next(it), next(it), next(it)
+        ('0', '1', '2', '3')
+        >>> list(it.elements())
+        ['2', '3']
+        >>> it.seek(0)
+        >>> next(it), next(it), next(it), next(it)
+        ('2', '3', '4', '5')
+        >>> next(it)
+        '6'
+
     """
 
-    def __init__(self, iterable):
+    def __init__(self, iterable, maxlen=None):
         self._source = iter(iterable)
-        self._cache = []
+        if maxlen is None:
+            self._cache = []
+        else:
+            self._cache = deque([], maxlen)
         self._index = None
 
     def __iter__(self):

more_itertools/more.pyi

@@ -272,7 +272,9 @@ class SequenceView(Generic[_T], Sequence[_T]):
     def __len__(self) -> int: ...
 
 class seekable(Generic[_T], Iterator[_T]):
-    def __init__(self, iterable: Iterable[_T]) -> None: ...
+    def __init__(
+        self, iterable: Iterable[_T], maxlen: Optional[int] = ...
+    ) -> None: ...
     def __iter__(self) -> seekable[_T]: ...
     def __next__(self) -> _T: ...
     def elements(self) -> SequenceView[_T]: ...

tests/test_more.py

@@ -2245,6 +2245,23 @@ def test_elements(self):
         mi.take(10, s)
         self.assertEqual(list(elements), [str(n) for n in range(20)])
 
+    def test_maxlen(self):
+        iterable = map(str, count())
+
+        s = mi.seekable(iterable, maxlen=4)
+        self.assertEqual(mi.take(10, s), [str(n) for n in range(10)])
+        self.assertEqual(list(s.elements()), ['6', '7', '8', '9'])
+
+        s.seek(0)
+        self.assertEqual(mi.take(14, s), [str(n) for n in range(6, 20)])
+        self.assertEqual(list(s.elements()), ['16', '17', '18', '19'])
+
+    def test_maxlen_zero(self):
+        iterable = [str(x) for x in range(5)]
+        s = mi.seekable(iterable, maxlen=0)
+        self.assertEqual(list(s), iterable)
+        self.assertEqual(list(s.elements()), [])
+
 
 class SequenceViewTests(TestCase):
     def test_init(self):