open with w and a, write/seek/truncate/tell

[agroce]

import pyfakefs.fake_filesystem
import os

fs = pyfakefs.fake_filesystem.FakeFilesystem()
fs.CreateDirectory('/Volumes')
fs.CreateDirectory('/Volumes/ramdisk')
fs.CreateDirectory('/Volumes/ramdisk/test')
os0 = pyfakefs.fake_filesystem.FakeOsModule(fs)
opener0 = pyfakefs.fake_filesystem.FakeFileOpen(fs)

component0 = "alpha"
path0 = "/Volumes/ramdisk/test"
path0 += "/" + component0
bytes0 = 0
bytes0 += 1
str0 = ""
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
file0 = opener0(path0,'w')
file1 = opener0(path0,'a')
file1.write(str0)
file0.seek(bytes0)
file0.truncate()
result = file1.tell()

print result

os 7 vs. pyfakefs 6

[mrbean-bremen]

Hm, can't reproduce this one. I get 6 in pyfakefs.

[agroce]

oops, reversed pyfakefs and os!

[agroce]

updated

[mrbean-bremen]

Hm, I don't understand why it shall be 7 - why not 6? After writing 6 bytes the position shall not have moved - what am I missing?

[agroce]

Not sure. This extension produces the same file contents, but the tell result is different.

component0 = "alpha"
path0 = "/Volumes/ramdisk/test"
path0 += "/" + component0
bytes0 = 0
bytes0 += 1
str0 = ""
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
str0 += 'a'
file0 = opener0(path0,'w')
file1 = opener0(path0,'a')
file1.write(str0)
file0.seek(bytes0)
file0.truncate()
file1.write('b')
file0.close()
file1.close()
file2 = opener0(path0,'r')
print file2.readline()

Hrm.

[mrbean-bremen]

Hmm. Have to understand this first, otherwise I have no idea how to fix it... I can reproduce the behavior under Windows (e.g. tell = 7), at least under Python 3, so there shall be a reason...

[mrbean-bremen]

I have still no real idea why this happens. It does not happen under Windows in Python 2, but happens in Python 3 (same as flush on reading, probably related). I happens only if file1 is open in append mode, file0 can be opened in both write or append mode. After the seek zeros are written to the file until the seek position, and the previously written content is appended to this. This happens the moment file1.tell() or file1.close() is called, e.g. on flush of file1.
Ok, that means that before flush() in append mode the position is reset to the current end of file - that makes some sense.
Note that I'm writing this more or less for my own reference, just thinking aloud... don't have a rubber duck here ;)

[mrbean-bremen]

Reopen as the fix broke other things.