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…ish() Since pythonGH-157861, every writer uses the read-only optimization of _PyUnicodeWriter_WriteStr(), so the first write of a str subclass instance into an empty writer kept that object as the buffer, and PyUnicodeWriter_Finish() returned it. io.StringIO.getvalue() then returned the written object itself, and the next write re-read it through its __str__() method, which changed the contents and could make read() read past the end of the buffer. Only use the read-only optimization for exact str objects. A subclass is copied into a new buffer, as before pythonGH-157861.
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_PyUnicodeWriter_WriteStr()now takes the read-only shortcut only for exactstrobjects, so astrsubclass is copied into a new buffer andPyUnicodeWriter_Finish()returns astr. This fixesio.StringIOafter writing astrsubclass.No NEWS entry: this fixes code added in GH-157861, which has not been released yet.
io.StringIOcorrupts its contents after writing astrsubclass #158213