René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`gzip._GzipReader.read()` with sometimes uninitialized variable","articleBody":"# Bug report\n\n### Bug description:\n\nThis one is easy to spot but hard to reproduce:\n\nhttps://github.com/python/cpython/blob/d7dbde895884d58e3da7ed4107fd33171afad7cb/Lib/gzip.py#L589\n\n`buf` is initialized only when `self._decompressor.needs_input`:\n\nhttps://github.com/python/cpython/blob/d7dbde895884d58e3da7ed4107fd33171afad7cb/Lib/gzip.py#L577\n\nIt exists since gh-95534 (#97664).\n\nI spent a lot of time trying to reproduce this bug using the standard zlib decompressor without any success:\n\n```\nimport gzip\nimport io\n\nclass EvilDecomp:\n    def __init__(self, **kwargs):\n        self.needs_input = False\n        self.unused_data = b\"\"\n        self.eof = False\n    def decompress(self, data, max_length):\n        self.needs_input = True\n        return b\"\"\n\ngzip.zlib._ZlibDecompressor = EvilDecomp\n\nwith gzip.GzipFile(\n    fileobj=io.BytesIO(gzip.compress(b\"hello world\")), mode=\"rb\"\n) as f:\n    f.read(1)\n```\n\nresults in:\n\n```\n22:16:40.678647000PM CEST maurycy@gimel /Users/maurycy/src/cpython % ./python.exe meow.py\nTraceback (most recent call last):\n  File \"/Users/maurycy/src/cpython/meow.py\", line 18, in \u003cmodule\u003e\n    f.read(1)\n    ~~~~~~^^^\n  File \"/Users/maurycy/src/cpython/Lib/gzip.py\", line 349, in read\n    return self._buffer.read(size)\n           ~~~~~~~~~~~~~~~~~^^^^^^\n  File \"/Users/maurycy/src/cpython/Lib/compression/_common/_streams.py\", line 68, in readinto\n    data = self.read(len(byte_view))\n  File \"/Users/maurycy/src/cpython/Lib/gzip.py\", line 589, in read\n    if buf == b\"\":\n       ^^^\nUnboundLocalError: cannot access local variable 'buf' where it is not associated with a value\n```\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-137572\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/maurycy","@type":"Person","name":"maurycy"},"datePublished":"2025-08-08T22:56:34.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/137571/cpython/issues/137571"}