Steven D'Aprano <steve+pyt...@pearwood.info> added the comment:
On Fri, Dec 25, 2020 at 01:31:51PM +0000, 施文峰 wrote:
> first test have a problem,you didn’t use r+ mode
I did, I copied your `test()` function exactly, however I did make a
mistake. I tried again with this:
>>> with open(FILE_PATH, 'r') as f:
... print(repr(f.read()))
...
'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00{"how_dare_you":
"how_dare_you"}'
so you are correct, the file is padded with NUL characters.
Here is a simpler demonstration of the behaviour.
```
FILE_PATH = 'example.data'
# create a file
with open(FILE_PATH, 'w') as f:
f.write('abc\n')
# Truncate using r+ mode.
with open(FILE_PATH, 'r+') as f:
assert f.tell() == 0
assert f.read() == 'abc\n'
assert f.tell() == 4 # File position is now at end of file.
f.truncate(0)
assert f.tell() == 4 # File position has not changed.
assert f.read() == '' # Nothing remaining to read.
f.write('xyz\n')
f.flush()
assert f.tell() == 8
assert f.read() == '' # Nothing remaining to read.
# Return the file position to start of file.
f.seek(0)
assert f.read() == '\0\0\0\0xyz\n'
```
All the assertions pass.
I think this is standard and correct behaviour. Do you have examples of
other programming languages that behave differently? PHP seems to do the
same thing:
https://www.php.net/manual/en/function.ftruncate.php
----------
title: io's r+ mode truncate(0) -> [issue] io's r+ mode truncate(0)
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<https://bugs.python.org/issue42733>
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