Propagate exceptions in sequence::size() (#2076)

tests/test_sequences_and_iterators.py

@@ -100,6 +100,25 @@ def test_sequence():
     assert cstats.move_assignments == 0
 
 
+def test_sequence_length():
+    """#2076: Exception raised by len(arg) should be propagated """
+    class BadLen(RuntimeError):
+        pass
+
+    class SequenceLike():
+        def __getitem__(self, i):
+            return None
+
+        def __len__(self):
+            raise BadLen()
+
+    with pytest.raises(BadLen):
+        m.sequence_length(SequenceLike())
+
+    assert m.sequence_length([1, 2, 3]) == 3
+    assert m.sequence_length("hello") == 5
+
+
 def test_map_iterator():
     sm = m.StringMap({'hi': 'bye', 'black': 'white'})
     assert sm['hi'] == 'bye'