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FileIO.seekable() can return False if we first seek to a position such
that, when truncated to a signed int, it becomes negative:
>>> f = open('largefile', 'wb', buffering=0)
>>> f.seek(2**31, 0)
2147483648
>>> f.write(b'x')
1
>>> f.close()
>>> f = open('largefile', 'rb', buffering=0)
>>> f.seek(0, 2)
2147483649
>>> f.seekable()
False

Patch with regression test in test_largefile: test 2**31-1, 2**31, 
2**31+1.

See also issue #5008.

Woops, I attached the wrong patch!

Committed in r70352 (py3k), r70353 (3.0). Needs backport to trunk and
2.6, if someone is interested.

> Needs backport to trunk and 2.6, if someone is interested.
I'm interrested by a backport at least in trunk. I updated the patch 
to trunk:
 - test directly _file._FileIO() instead of using open() because in 
Python trunk, a file has no seekable() method :-/
 - use contextlib.closing() because _FileIO doesn't implement context 
API (not __exit__() method)

You can use io.open() instead of open()...

> You can use io.open() instead of open()...
Alright, it's much easier with io.open() :-)

It is fixed in 2.7 with the backport of the Python3's io library (r73394).