René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"email.policy.default adds an extra newline char to email address headers and raises HeaderWriteError on stringify","articleBody":"# Bug report\n\n### Bug description:\n\nReproducer script:\n\n```python\n#!/usr/bin/env python3\n\nimport email\nimport email.policy\n\nmsg_bytes = b\"\"\"\\\nTo: loooooooooooooooooooooooooooooooooooong@dooooooooooooomainname.example.com,\n xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.example.com\n\nbody\n\"\"\"\n\nmsg = email.message_from_bytes(msg_bytes, policy=email.policy.default)\nprint(msg['to'])\nprint(msg)\n```\n\nOutput:\n\n```console\n$ python3.14 test.py\nloooooooooooooooooooooooooooooooooooong@dooooooooooooomainname.example.com, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.example.com\nTraceback (most recent call last):\n  File \"/home/fumiyas/test.py\", line 15, in \u003cmodule\u003e\n    print(msg)\n    ~~~~~^^^^^\n  File \"/usr/lib/python3.14/email/message.py\", line 1009, in __str__\n    return self.as_string(policy=self.policy.clone(utf8=True))\n           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/usr/lib/python3.14/email/message.py\", line 1006, in as_string\n    return super().as_string(unixfrom, maxheaderlen, policy)\n           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/usr/lib/python3.14/email/message.py\", line 188, in as_string\n    g.flatten(self, unixfrom=unixfrom)\n    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/usr/lib/python3.14/email/generator.py\", line 117, in flatten\n    self._write(msg)\n    ~~~~~~~~~~~^^^^^\n  File \"/usr/lib/python3.14/email/generator.py\", line 200, in _write\n    self._write_headers(msg)\n    ~~~~~~~~~~~~~~~~~~~^^^^^\n  File \"/usr/lib/python3.14/email/generator.py\", line 234, in _write_headers\n    raise HeaderWriteError(\n        f'folded header contains newline: {folded!r}')\nemail.errors.HeaderWriteError: folded header contains newline: 'To: loooooooooooooooooooooooooooooooooooong@dooooooooooooomainname.example.com,\\n\\n xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.example.com\\n'\n```\n\nNo problem with `email.policy.compat32`.\n\n### CPython versions tested on:\n\n3.13, 3.14, 3.12, 3.11, 3.10, 3.9\n\n### Operating systems tested on:\n\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-142008\n* gh-142361\n* gh-142362\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/fumiyas","@type":"Person","name":"fumiyas"},"datePublished":"2025-11-27T09:29:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/142006/cpython/issues/142006"}