make 'enumerate_reversed()' work with generators (#2795)

gallery_dl/util.py

@@ -161,13 +161,16 @@ def delete_items(obj, keys):
 
 def enumerate_reversed(iterable, start=0, length=None):
     """Enumerate 'iterable' and return its elements in reverse order"""
-    start -= 1
     if length is None:
         length = len(iterable)
-    return zip(
+
-        range(length - start, start, -1),
+    try:
-        reversed(iterable),
+        iterable = zip(range(start-1+length, start-1, -1), reversed(iterable))
-    )
+    except TypeError:
+        iterable = list(zip(range(start, start+length), iterable))
+        iterable.reverse()
+
+    return iterable
 
 
 def number_to_string(value, numbers=(int, float)):

test/test_util.py

@@ -541,6 +541,41 @@ class TestOther(unittest.TestCase):
         r = util.filter_dict(d)
         self.assertEqual(r, {"foo": 123})
 
+    def test_enumerate_reversed(self):
+
+        seq = [11, 22, 33]
+        result = [(3, 33), (2, 22), (1, 11)]
+
+        def gen():
+            for i in seq:
+                yield i
+
+        def gen_2():
+            yield from seq
+
+        def assertEqual(it1, it2):
+            ae = self.assertEqual
+            for i1, i2 in zip(it1, it2, strict=True):
+                ae(i1, i2)
+
+        assertEqual(
+            util.enumerate_reversed(seq), [(2, 33), (1, 22), (0, 11)])
+        assertEqual(
+            util.enumerate_reversed(seq, 1), result)
+        assertEqual(
+            util.enumerate_reversed(seq, 2), [(4, 33), (3, 22), (2, 11)])
+
+        assertEqual(
+            util.enumerate_reversed(gen(), 0, len(seq)),
+            [(2, 33), (1, 22), (0, 11)])
+        assertEqual(
+            util.enumerate_reversed(gen(), 1, len(seq)), result)
+        assertEqual(
+            util.enumerate_reversed(gen_2(), 1, len(seq)), result)
+        assertEqual(
+            util.enumerate_reversed(gen_2(), 2, len(seq)),
+            [(4, 33), (3, 22), (2, 11)])
+
     def test_number_to_string(self, f=util.number_to_string):
         self.assertEqual(f(1)     , "1")
         self.assertEqual(f(1.0)   , "1.0")