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

2016-06-08 Thread 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

2016-06-07 Thread 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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