[issue1539381] Add readinto method to StringIO and cStringIO

[Alan Justino]

Alan Justino <alan.justino at yahoo.com.br> added the comment:

It affects SpooledTemporaryFile too. Because it switches between StringIO, BytesIO and TemporaryFile internally. Only StringIO have not this interface.

Discovered trying to `pickle.load` a remote storage file.

Please reopen.

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nosy: +alanjds

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue1539381>
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