[New-bugs-announce] [issue35304] The return of truncate(size=None) (file io) is unexpected

[liugang]

New submission from liugang <liugang93 at 163.com>:

-- run in Windows 10

1 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # output 5 (xxx\r\n)

x = f.truncate()
print(x)  # output 13 (xxx\r\nyyy\r\nzzz), why it is 13, not 5?

2 - 
f = open('test', "w+")
f.write('xxx\nyyy\nzzz')

f.seek(0)

f.readline()
print(f.tell())  # output 5 (xxx\r\n)

f.seek(f.tell())

x = f.truncate()
print(x)  # output 5

----------
components: IO
messages: 330357
nosy: liugang93
priority: normal
severity: normal
status: open
title: The return of truncate(size=None) (file io) is unexpected
type: behavior
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35304>
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