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Document itertools.tee's internal batching #154369

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8 changes: 8 additions & 0 deletions Doc/library/itertools.rst
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Expand Up @@ -757,6 +757,14 @@ loops that truncate the stream.
most or all of the data before another iterator starts, it is faster to use
:func:`list` instead of :func:`tee`.

.. impl-detail::

Internally, CPython's :func:`tee` groups cached items into fixed-size
batches (57 items each currently) and only frees a batch once all
iterators have consumed it. Memory usage is thus bounded by the gap
between the fastest and slowest iterator plus up to one batch, not by
that gap alone.


.. function:: zip_longest(*iterables, fillvalue=None)

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