Add an utils.newest_slice.

This takes an iterable and uses itertools.islice to get the n newest elements
from it.

qutebrowser/test/utils/test_utils.py

@@ -404,5 +404,57 @@ class ForceEncodingTests(unittest.TestCase):
         self.assertEqual(utils.force_encoding(text, 'ascii'), 'hell? w?rld')
 
 
+class NewestSliceTests(unittest.TestCase):
+
+    """Test newest_slice."""
+
+    def test_count_minus_two(self):
+        """Test with a count of -2."""
+        with self.assertRaises(ValueError):
+            utils.newest_slice([], -2)
+
+    def test_count_minus_one(self):
+        """Test with a count of -1 (all elements)."""
+        items = range(20)
+        sliced = utils.newest_slice(items, -1)
+        self.assertEqual(list(sliced), list(items))
+
+    def test_count_zero(self):
+        """Test with a count of 0 (no elements)."""
+        items = range(20)
+        sliced = utils.newest_slice(items, 0)
+        self.assertEqual(list(sliced), [])
+
+    def test_count_much_smaller(self):
+        """Test with a count which is much smaller than the iterable."""
+        items = range(20)
+        sliced = utils.newest_slice(items, 5)
+        self.assertEqual(list(sliced), [15, 16, 17, 18, 19])
+
+    def test_count_smaller(self):
+        """Test with a count which is exactly one smaller."""
+        items = range(5)
+        sliced = utils.newest_slice(items, 4)
+        self.assertEqual(list(sliced), [1, 2, 3, 4])
+
+    def test_count_equal(self):
+        """Test with a count which is just as large as the iterable."""
+        items = range(5)
+        sliced = utils.newest_slice(items, 5)
+        self.assertEqual(list(sliced), list(items))
+
+    def test_count_bigger(self):
+        """Test with a count which is one bigger than the iterable."""
+        items = range(5)
+        sliced = utils.newest_slice(items, 6)
+        self.assertEqual(list(sliced), list(items))
+
+    def test_count_much_bigger(self):
+        """Test with a count which is much bigger than the iterable."""
+        items = range(5)
+        sliced = utils.newest_slice(items, 50)
+        self.assertEqual(list(sliced), list(items))
+
+
 if __name__ == '__main__':
     unittest.main()

qutebrowser/utils/utils.py

@@ -27,6 +27,7 @@ import os.path
 import collections
 import functools
 import contextlib
+import itertools
 
 from PyQt5.QtCore import Qt
 from PyQt5.QtGui import QKeySequence, QColor
@@ -552,3 +553,22 @@ def force_encoding(text, encoding):
     This replaces all chars not encodable with question marks.
     """
     return text.encode(encoding, errors='replace').decode(encoding)
+
+
+def newest_slice(iterable, count):
+    """Get an iterable for the n newest items of the given iterable.
+
+    Args:
+        count: How many elements to get.
+               0: get no items:
+               n: get the n newest items
+              -1: get all items
+    """
+    if count < -1:
+        raise ValueError("count can't be smaller than -1!")
+    elif count == 0:
+        return []
+    elif count == -1 or len(iterable) < count:
+        return iterable
+    else:
+        return itertools.islice(iterable, len(iterable) - count, len(iterable))