# AN-2026-0761: Python 3.15 (PEP 686): UTF-8 is the default encoding for open() and other I/O without an explicit encoding, regardless of locale (Windows cp1252 code changes behavior)

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- id: AN-2026-0761 (version 1; cite https://breakwire.org/claim/AN-2026-0761/v/1)
- kind: breaking · status: active · review: editor
- source: <https://docs.python.org/3/whatsnew/3.15.html#:~:text=Python%20now%20uses%20UTF%2D8%20as%20the%20default%20encoding>
- more sources: <https://www.python.org/downloads/>
- event date: 2026-10-09
- services: python
- affected: engines.python >=3.15
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. What's New in Python 3.15 (opened 2026-11-04): Python now uses UTF-8 as the default encoding, independent of the system's environment, so I/O operations without an explicit encoding (for example open('flying-circus.txt')) use UTF-8. This only applies when no encoding argument is given; pass an explicit encoding for compatibility across versions.  [✓0 ✗0]
1. The same page says encoding='locale' keeps using the current locale encoding (supported since 3.10) and that UTF-8 mode can be disabled with PYTHONUTF8=0 or -X utf8=0 to retain the previous behavior. Checked 2026-11-04 on CPython 3.15.2 (Linux): sys.flags.utf8_mode = 1 and locale.getpreferredencoding(False) = utf-8.  [✓0 ✗0]
2. Most visible on Windows, where the default was the ANSI code page (e.g. cp1252): files written by older Pythons in that code page can now raise UnicodeDecodeError or read as mojibake.  [✓0 ✗0]

## Fix
```text
Pass encoding= explicitly on open(), Path.read_text()/write_text(), subprocess text mode and similar; use encoding='locale' where the locale encoding is really wanted; run with PYTHONUTF8=0 (or -X utf8=0) as a temporary opt-out; enable -X warn_default_encoding on older versions to find affected calls.
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0761/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28