[ARROW-3228] [Python] Immutability of bytes is ignored

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.11.0
• Component/s: Python
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/19572
• Assignee: Wes McKinney
• Reporter: Pearu Peterson
• Created: 12/Sep/18 20:33
• Updated: 11/Jan/23 07:26
• Resolved: 16/Sep/18 00:43

Description

Creating a pyarrow.Buffer from Python bytes allows in-place changes of immutable Python strings:

>>> import pyarrow as pa
>>> import numpy as np
>>> a = b'123456'
>>> a[0] = 77 # bytes are immutable, so TypeError is expected
TypeError: 'bytes' object does not support item assignment
>>> b = pa.py_buffer(a)  # but with pyarrow bytes can be changed in-place
>>> arr = np.frombuffer(b, dtype=np.uint8)
>>> arr[0] = 66 # change 'a' in-place, would expect error
>>> a
b'B23456'
>>> hash(a)
-4581532003987476523
>>> arr[0] = 77
>>> a
b'M23456'
>>> hash(a) # hash value stays constant while changing 'a'
-4581532003987476523

Notice that numpy.frombuffer respects immutability of bytes:
{{}}

>>> arr2 = np.frombuffer(a, dtype=np.uint8)
>>> arr2
array([77, 50, 51, 52, 53, 54], dtype=uint8)
>>> arr2[0] = 88 # expecting error
ValueError: assignment destination is read-only

Issue Links

• links to: GitHub Pull Request #2567