fixes

src/lightning/pytorch/loops/fetchers.py

@@ -48,8 +48,8 @@ def setup(self, combined_loader: CombinedLoader) -> None:
         # self.done = self.length == 0
 
     def __iter__(self) -> "_DataFetcher":
-        self.reset()
         self.iterator = iter(self.combined_loader)
+        self.reset()
         return self
 
     def __next__(self) -> Any:

src/lightning/pytorch/utilities/combined_loader.py

@@ -108,8 +108,11 @@ class _MinSize(_ModeIterator[List]):
     def __next__(self) -> List:
         return [next(it) for it in self.iterators]
 
-    # def __len__(self) -> Optional[int]:
-    #     return min(self.limits) if self.limits is not None else None
+    def __len__(self) -> Optional[int]:
+        lengths = _get_iterables_lengths(self.iterables)
+        if self.limits is not None:
+            return min([min(length, limit) for length, limit in zip(lengths, self.limits)])
+        return min(lengths)
 
 
 class _Sequential(_ModeIterator[Tuple[Any, int, int]]):
@@ -355,8 +358,9 @@ def _shutdown_workers_and_reset_iterator(dataloader: object) -> None:
 def _get_iterables_lengths(iterables):
     lengths = []
     for iterable in iterables:
-        length = sized_len(iterable)
-        if length is None:
-            raise NotImplementedError(f"`{type(iterable).__name__}` does not define `__len__`")
+        if (length := sized_len(iterable)) is None:
+            length = float("inf")
+        # if length is None:
+        #     raise NotImplementedError(f"`{type(iterable).__name__}` does not define `__len__`")
         lengths.append(length)
     return lengths

tests/tests_pytorch/loops/test_fetchers.py

@@ -540,3 +540,22 @@ def test_done_dataloader_iter(iterable):
     with pytest.raises(StopIteration):
         next(dataloader_iter)
     assert dataloader_iter.done
+
+
+@pytest.mark.parametrize("iterable", [[1, 2, 3], IterDataset()])
+def test_done_dataloader_iter_with_limit(iterable):
+    loader = CombinedLoader(iterable)
+    fetcher = _DataLoaderIterDataFetcher()
+    fetcher.setup(loader)
+    iter(fetcher)
+    loader._iterator.limits = [1]
+    fetcher.reset()
+
+    assert not fetcher.done
+    dataloader_iter = next(fetcher)
+    assert not dataloader_iter.done
+    assert next(dataloader_iter) == 1
+    assert dataloader_iter.done
+    assert fetcher.done
+    # with pytest.raises(StopIteration):  # TODO why is it not raising?
+    #     next(dataloader_iter)