io.BufferedReader.writabe is False, but io.BufferedReader.truncate does not raise OSError

[steverpalmer]

BPO	35950
Nosy	@berkerpeksag, @vadmium, @miss-islington, @steverpalmer
PRs	bpo-35950: Raise OSError in io.BufferedReader.truncate() #13689 bpo-35950: Raise UnsupportedOperation in BufferedReader.truncate() #18586

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2020-02-21.18:01:25.803>
created_at = <Date 2019-02-09.13:36:22.181>
labels = ['3.9', 'expert-IO']
title = 'io.BufferedReader.writabe is False, but io.BufferedReader.truncate does not raise OSError'
updated_at = <Date 2020-02-22.07:19:05.846>
user = 'https://github.com/steverpalmer'

bugs.python.org fields:

activity = <Date 2020-02-22.07:19:05.846>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2020-02-21.18:01:25.803>
closer = 'methane'
components = ['IO']
creation = <Date 2019-02-09.13:36:22.181>
creator = 'steverpalmer'
dependencies = []
files = []
hgrepos = []
issue_num = 35950
keywords = ['patch']
message_count = 2.0
messages = ['335132', '362417']
nosy_count = 4.0
nosy_names = ['berker.peksag', 'martin.panter', 'miss-islington', 'steverpalmer']
pr_nums = ['13689', '18586']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35950'
versions = ['Python 3.9']

[steverpalmer]

An io.BUfferedReader object has an (inherited) writable method that returns False. io.IOBase states in the description of the writable method that "If False, write() and truncate() will raise OSError."

However, if the BufferedReader object is constructed from a writabe io.RawIOBase object, then the truncate does not raise the exception.

>>> import io
>>> import tempfile
>>> rf = tempfile.TemporaryFile(buffering=0)
>>> rf
<_io.FileIO name=3 mode='rb+' closefd=True>
>>> bf = io.BufferedReader(rf)
>>> bf.writable()
False
>>> bf.truncate(0)
0

Looking at _pyio.py file, the truncate method in the _BufferedIOMixin wrapper class delegates the truncation to it's raw attribute. If the raw element permits the truncation, then it will proceed regardless of the writable state of the wrapper class. I'd suggest that modifying the truncate method in _BufferedIOMixin to raise OSError (or Unsupported) if not self.writable() could fix this problem.

[miss-islington]

New changeset fd5116c by Berker Peksag in branch 'master':
bpo-35950: Raise UnsupportedOperation in BufferedReader.truncate() (GH-18586)
fd5116c