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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"GzipFile.seek makes invalid write if buffer is not flushed in Python 3.12rc1","articleBody":"# Bug report\r\n\r\n### Checklist\r\n\r\n- [X] I am confident this is a bug in CPython, not a bug in a third-party project\r\n- [X] I have searched the [CPython issue tracker](https://github.com/python/cpython/issues?q=is%3Aissue+sort%3Acreated-desc),\r\nand am confident this bug has not been reported before\r\n\r\n\r\n### CPython versions tested on:\r\n\r\n3.12\r\n\r\n### Operating systems tested on:\r\n\r\nLinux\r\n\r\n### Output from running 'python -VV' on the command line:\r\n\r\nPython 3.12.0rc1 (main, Aug 16 2023, 05:03:59) [GCC 12.2.0]\r\n\r\n### A clear and concise description of the bug:\r\n\r\nI have code that writes out sections of a data file in chunks, and uses seeks to ensure that the position is correct before writing.\r\n\r\nIn the following example, I write 5 bytes, seek to position 5 and write five more bytes. If I flush the buffer, the result is as expected. If I do not, 5 null bytes are written between the two groups of intended bytes.\r\n\r\n```python\r\n#!/usr/bin/env python\r\n\r\nimport io\r\nimport gzip\r\n\r\n\r\nfor flush in (True, False):\r\n    data = io.BytesIO()\r\n    gzip_writer = gzip.GzipFile(fileobj=data, mode='wb')\r\n    gzip_writer.write(b'abcde')\r\n\r\n    # If the buffer isn't flushed, seek works from unchanged offset\r\n    if flush and hasattr(gzip_writer, '_buffer'):\r\n        gzip_writer._buffer.flush()\r\n\r\n    gzip_writer.seek(5)\r\n    gzip_writer.write(b'fghij')\r\n    gzip_writer.close()\r\n    \r\n    # Recover result\r\n    data.seek(0)\r\n    gzip_reader = gzip.GzipFile(fileobj=data, mode='rb')\r\n    result = gzip_reader.read()\r\n\r\n    print(f'{flush=}: {result}')\r\n\r\n```\r\n\r\nIn the case where I seek but don't tell, I get spurious `\\x00` bytes:\r\n\r\n```\r\nflush=True: b'abcdefghij'\r\nflush=False: b'abcde\\x00\\x00\\x00\\x00\\x00fghij'\r\n```\r\n\r\nHere is the output in Python 3.10.10:\r\n\r\n```\r\nflush=True: b'abcdefghij'\r\nflush=False: b'abcdefghij'\r\n```\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-108341\n* gh-108402\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/effigies","@type":"Person","name":"effigies"},"datePublished":"2023-08-18T14:14:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/108111/cpython/issues/108111"}