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io.BytesIO.truncate does not work as advertised #71448
Description
| BPO | 27261 |
|---|---|
| Nosy | @avassalotti, @vadmium, @serhiy-storchaka, @swarmer, @zhangyangyu, @teythoon, @tirkarthi |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2016-06-07.20:52:57.361>
labels = ['type-bug', 'library', 'expert-IO']
title = 'io.BytesIO.truncate does not work as advertised'
updated_at = <Date 2018-10-12.10:55:18.233>
user = 'https://github.com/teythoon'bugs.python.org fields:
activity = <Date 2018-10-12.10:55:18.233>
actor = 'anton.barkovsky'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'IO']
creation = <Date 2016-06-07.20:52:57.361>
creator = 'justus.winter'
dependencies = []
files = []
hgrepos = []
issue_num = 27261
keywords = []
message_count = 5.0
messages = ['267748', '267758', '267765', '267848', '327578']
nosy_count = 7.0
nosy_names = ['alexandre.vassalotti', 'martin.panter', 'serhiy.storchaka', 'anton.barkovsky', 'xiang.zhang', 'justus.winter', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27261'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']
Activity
teythoon commentedon Jun 7, 2016
% python3.6 -c "import io; b=io.BytesIO(); assert b.truncate(42) == 42; assert len(b.getbuffer()) == 42, 'expected length 42, got {}'.format(len(b.getbuffer()))" Traceback (most recent call last): File "<string>", line 1, in <module> AssertionError: expected length 42, got 0The documentation [0] says that truncate can extend objects, and there is no indication that BytesIO does not support that. As demonstrated, truncate returns the new size, but the buffer obtained from that BytesIO is of size zero (likewise, b.getvalue() returns b''.
0: https://docs.python.org/3.6/library/io.html#io.IOBase.truncate
vadmium commentedon Jun 8, 2016
BytesIO is supposed to implement IOBase. I would treat this as a bug in existing versions too.
zhangyangyu commentedon Jun 8, 2016
Is this a bug or a documentation matter? Inspecting BytesIO.truncate's code, it does resizing similar to list, this seems to be a designed feature, which conflicts with the documented behaviour.
teythoon commentedon Jun 8, 2016
FWIW, I consider the documented behavior a feature. My use case is to allocate a BytesIO object with a given size, obtain a view to its buffer, and write to it from a c library.
swarmer commentedon Oct 12, 2018
I'm willing to try to fix this behavior.
I just want to check that this would not be considered breaking backwards compatibility. I can imagine in theory some code relying on it, but I would say that it would be relying on a bug. If some code is passed BytesIO in place of a file, then the current behavior is clearly undesirable. If some code specifically uses BytesIO and relies on this... I guess this can happen, but should be very rare and contrary to widely documented behavior. So it seems ok to just fix this, but I'm not very familiar with how such changes are usually handled in cPython, so I'd like to get approval from someone experienced.
RayKoopa commentedon Jun 15, 2026
Still an issue.
truncatereturns a nonsensical result that does not represent its new buffer size. So even if you check it - you're being lied to.cmaloney commentedon Jun 17, 2026
I built a prototype of this new feature: https://github.com/python/cpython/compare/main...cmaloney:cpython:gh_71488_exp?expand=1.
@serhiy-storchaka if that looks like a reasonable change to you I'll add the NEWS and work into a full PR.
cmaloney commentedon Jun 30, 2026
Implemented the missing behavior, truncate which extends, in #152252 which is ready for review.