[issue27261] io.BytesIO.truncate does not work as advertised

[Anton Barkovsky]

Anton Barkovsky  added the comment:

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.

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[issue27261] io.BytesIO.truncate does not work as advertised

[Anton Barkovsky]

Change by Anton Barkovsky :


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nosy: +anton.barkovsky

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[issue27261] io.BytesIO.truncate does not work as advertised

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +xtreak

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[issue27261] io.BytesIO.truncate does not work as advertised

[Justus Winter]

Justus Winter added the comment:

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.

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[issue27261] io.BytesIO.truncate does not work as advertised

[Xiang Zhang]

Changes by Xiang Zhang :


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nosy: +alexandre.vassalotti, serhiy.storchaka

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[issue27261] io.BytesIO.truncate does not work as advertised

[Xiang Zhang]

Xiang Zhang added the comment:

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.

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nosy: +xiang.zhang

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[issue27261] io.BytesIO.truncate does not work as advertised

[Martin Panter]

Martin Panter added the comment:

BytesIO is supposed to implement IOBase. I would treat this as a bug in 
existing versions too.

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nosy: +martin.panter
stage:  -> needs patch
versions: +Python 2.7, Python 3.5

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[issue27261] io.BytesIO.truncate does not work as advertised

[Justus Winter]

New submission from Justus Winter:

% 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 "", line 1, in 
AssertionError: expected length 42, got 0

The 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

--
components: IO, Library (Lib)
messages: 267748
nosy: justus.winter
priority: normal
severity: normal
status: open
title: io.BytesIO.truncate does not work as advertised
type: behavior
versions: Python 3.6

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