Micro-optimization for LazyJsonlIterator len() (#1237)

lhotse/lazy.py

@@ -182,10 +182,14 @@ def __init__(self, path: Pathlike) -> None:
         self._len = None
 
     def __iter__(self):
+        tot = 0
         with open_best(self.path, "r") as f:
             for line in f:
                 data = decode_json_line(line)
                 yield data
+                tot += 1
+        if self._len is None:
+            self._len = tot
 
     def __len__(self) -> int:
         if self._len is None:

test/test_lazy.py

@@ -9,6 +9,7 @@
 import pytest
 
 from lhotse import CutSet, FeatureSet, RecordingSet, SupervisionSet, combine
+from lhotse.lazy import LazyJsonlIterator
 from lhotse.testing.dummies import DummyManifest, as_lazy
 from lhotse.utils import fastcopy, is_module_available
 
@@ -248,3 +249,18 @@ def test_dillable():
             "dummy-mono-cut-0000-random-suffix",
             "dummy-mono-cut-0001-random-suffix",
         ]
+
+
+def test_lazy_jsonl_iterator_caches_len():
+    cuts = DummyManifest(CutSet, begin_id=0, end_id=200)
+    expected_len = 200
+    with as_lazy(cuts) as cuts_lazy:
+        path = cuts_lazy.data.path
+        print(path)
+        it = LazyJsonlIterator(path)
+        assert it._len is None
+        for _ in it:
+            pass
+        assert it._len is not None
+        assert it._len == expected_len
+        assert len(it) == expected_len