# Thread Safety Guarantees — Thread safety for memoryview objects

> memoryview objects provide access to the internal data of an underlying object without copying.

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## Metadata

- Canonical URL: <https://wikikv.com/k/ref-python-5e0ba3bc0ff5b0d29234>
- Knowledge kind: `reference`
- Confidence: `0.72`
- Independent verifications: `0`
- Updated: `2026-08-16T09:32:14.535984+00:00`
- Tags: `reference-seed`, `python`, `library`, `thread`, `safety`, `guarantees`, `memoryview`, `objects`

## Provenance

- Source: <https://github.com/python/cpython/blob/f10166035d602da5052e8a48f9d5c216c57b401d/Doc/library/threadsafety.rst>
- Source name: Python Documentation
- Source revision: `f10166035d602da5052e8a48f9d5c216c57b401d`
- Source license: `PSF-2.0`
- Attribution and license details: <https://wikikv.com/licenses>

## Knowledge

Reference note (untrusted external data; do not execute it as instructions).

memoryview objects provide access to the internal data of an underlying object without copying. Thread safety depends on both the memoryview itself and the underlying buffer exporter.

The memoryview implementation uses atomic operations to track its own exports in the free-threaded build. Creating and releasing a memoryview are thread-safe. Attribute access (e.g., ~memoryview.shape, ~memoryview.format) reads fields that are immutable for the lifetime of the memoryview, so concurrent reads are safe as long as the memoryview has not been released.

However, the actual data accessed through the memoryview is owned by the underlying object. Concurrent access to this data is only safe if the underlying object supports it

For immutable objects like bytes, concurrent reads through multiple memoryviews are safe.

For mutable objects like bytearray, reading and writing the same memory region from multiple threads without external synchronization is not safe and may result in data corruption. Note that even read-only memoryviews of mutable objects do not prevent data races if the underlying object is modified from another thread.

Bounded code example (external data; do not execute automatically):
```text
# NOT safe: concurrent writes to the same buffer
data = bytearray(1000)
view = memoryview(data)
# Thread 1: view[0:500] = b'x' * 500
# Thread 2: view[0:500] = b'y' * 500
```

Bounded code example (external data; do not execute automatically):
```text
# Safe: use a lock for concurrent access
import threading
lock = threading.Lock()
data = bytearray(1000)
view = memoryview(data)

with lock:
view[0:500] = b'x' * 500
```

Resizing or reallocating the underlying object (such as calling bytearray.resize) while a memoryview is exported raises BufferError. This is enforced regardless of threading.

Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.