def _iter_impl(self):
        """
        The Data Loader iterator stops the for-loop when reader runs out of samples.
        """
        # As we iterate over incoming samples, we are going to store them in `self._batch_acc`, until we have a batch of
        # the requested batch_size ready.

        keys = None
        if self.shuffling_queue_capacity > 0:
            # We can not know what is the reasonable number to use for the extra capacity, so we set a huge number
            # and give up on the unbound growth protection mechanism.
            min_after_dequeue = self.shuffling_queue_capacity - 1
            self._shuffling_buffer = RandomShufflingBuffer(self.shuffling_queue_capacity,
                                                           min_after_retrieve=min_after_dequeue,
                                                           extra_capacity=100000000)
        else:
            self._shuffling_buffer = NoopShufflingBuffer()

        for row in self.reader:
            # Default collate does not work nicely on namedtuples and treat them as lists
            # Using dict will result in the yielded structures being dicts as well
            row_as_dict = row._asdict()

            keys = row_as_dict.keys()

            # Promote some types that are incompatible with pytorch to be pytorch friendly.
            _sanitize_pytorch_types(row_as_dict)

            # Add rows to shuffling buffer
            if not self.reader.batched_output:
                self._shuffling_buffer.add_many([row_as_dict])
            else:
                # Transposition:
                #   row_as_dict:        {'a': [1,2,3], 'b':[4,5,6]}
                #   row_group_as_tuple: [(1, 4), (2, 5), (3, 6)]
                # The order within a tuple is defined by key order in 'keys'
                row_group_as_tuple = list(zip(*(row_as_dict[k] for k in keys)))

                # Adding data as 'row-by-row' into a shuffling buffer. This is a pretty
                # slow implementation though. Probably can comeup with a faster way to shuffle,
                # perhaps at the expense of a larger memory consumption...
                self._shuffling_buffer.add_many(row_group_as_tuple)

            # _yield_batches will emit as much batches as are allowed by the shuffling_buffer (RandomShufflingBuffer
            # will avoid underflowing below a certain number of samples to guarantee some samples decorrelation)
            for batch in self._yield_batches(keys):
                yield batch

        # Once reader can not read new rows, we might still have a bunch of rows waiting in the shuffling buffer.
        # Telling shuffling buffer that we are finished allows to deplete the buffer completely, regardless its
        # min_after_dequeue setting.
        self._shuffling_buffer.finish()

        for batch in self._yield_batches(keys):
            yield batch

        # Yield the last and partial batch
        if self._batch_acc:
            yield self.collate_fn(self._batch_acc)

        The Data Loader iterator stops the for-loop when reader runs out of samples.
        
petastorm/pytorch.py