Implement optimized iteration in B2Dataset.

And adapt the related tests.  Also remove the readme notice about the
operation not being optimized.

Fixes #2.

README.rst

@@ -33,13 +33,6 @@ Finally, you may explicitly enable optimizations for a given h5py dataset by wra
     b2dset = b2h5py.B2Dataset(dset)
     # ... slicing ``b2dset`` will use Blosc2 optimization ...
 
-Please note that, for the moment, plain iteration in ``B2Dataset`` instances is not optimized (as it falls back to plain ``Dataset`` slicing). This does not affect the other approaches further above. Instead of ``for row in b2dset:`` loops, you may prefer to use slicing like::
-
-    for i in range(len(b2dset)):
-        # ... operate with ``b2dset[i]`` or ``b2dset[i, ...]`` ...
-
-We recommend that you test which approach works better for your datasets. This limitation may be fixed in the future.
-
 Building
 --------
 

b2h5py/blosc2.py

@@ -235,6 +235,16 @@ def is_b2_fast_slicing(self) -> bool:
         """Whether or not Blosc2 optimized slicing is enabled"""
         return getattr(self, opt_dataset_ok_prop)
 
+    def __iter__(self):
+        # This needs to be reimplemented here,
+        # lest the base dataset iteration is called
+        # which uses its getitem, not ours.
+        shape = self.__dataset.shape
+        if len(shape) < 1:
+            return iter(self.__dataset)  # scalar, let it fail
+        for row in range(shape[0]):
+            yield self[row]
+
     def __getitem__(self, args):
         try:
             selection = opt_slice_check(self, args)

b2h5py/tests/test_b2dataset.py

@@ -36,11 +36,10 @@ def testB2Dataset(self):
         self.assertArrayEqual(b2dataset[::2], self.arr[::2])
 
     def testIter(self):
-        """Iteration does not hang"""
+        """Iteration does use optimization"""
         b2dataset = B2Dataset(self.dset)
         self.assertTrue(b2dataset.is_b2_fast_slicing)
 
-        b2dsiter = iter(b2dataset)
-        next(b2dsiter)
-        next(b2dsiter)
-        return
+        with checking_opt_slicing():
+            for (b2row, arow) in zip(b2dataset, self.arr):
+                self.assertArrayEqual(b2row, arow)