# AN-2026-0765: NumPy 2.3 (Jun 7, 2025) removed ndarray.tostring(): "'numpy.ndarray' object has no attribute 'tostring'"

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- id: AN-2026-0765 (version 1; cite https://breakwire.org/claim/AN-2026-0765/v/1)
- kind: breaking · status: active · review: editor
- source: <https://numpy.org/doc/stable/release/2.3.0-notes.html#:~:text=has%20been%20removed%2C%20use%20tobytes%20instead>
- more sources: <https://github.com/numpy/numpy/blob/v2.5.4/doc/source/release/2.3.0-notes.rst> <https://github.com/python/cpython/blob/v3.15.2/Objects/object.c>
- event date: 2025-06-07
- services: numpy, python
- affected: pypi:numpy >=2.3.0
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The NumPy 2.3.0 release notes (opened 2026-11-04), Expired deprecations: np.tostring has been removed, use tobytes instead (deprecated since 1.19); the same release turns other long-deprecated behaviour into errors, e.g. np.bincount(..., minlength=None), shape=None for required shapes, inexact mode/searchside matches, and np.fromstring binary mode (use np.frombuffer).  [✓0 ✗0]
1. Reproduced 2026-11-04: numpy 2.0.2 emits DeprecationWarning "tostring() is deprecated. Use tobytes() instead."; numpy 2.4.4 and 2.5.4 raise AttributeError: 'numpy.ndarray' object has no attribute 'tostring'.  [✓0 ✗0]
2. numpy 2.3.0 was uploaded to PyPI on 2025-06-07.  [✓0 ✗0]

## Fix
```text
Use arr.tobytes() (same bytes, same order argument). Use np.frombuffer instead of np.fromstring for binary data, and minlength=0 for bincount.
```

## Error signatures
- code: `'numpy.ndarray' object has no attribute 'tostring'` (CPython attribute error; reproduced on numpy 2.4.4 / 2.5.4)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0765/reports_

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