Fix pickling of std(in, out, err) streams

[matsjoyce]

Fixes one of the problems in pickling sys as encounted in #41. As you cannot file.tell() a standard stream, this pr stops dill from trying and failing with an IOError.

[matsjoyce]

I don't think that files ever have a negative tell()

[mmckerns]

Not sure setting the position = -1 is the right thing to do.

For sys.stdin, it seems the position rolls forward with time, on it's own (starting from 0?). It also seems that stdin retains a queue -- as long as you don't seek back to a different position.

>>> f = sys.stdin
>>> f.read(3)
koaksdcpoaskovkas
'koa'
>>> f.read(3)
'ksd'
>>> f.read(3)
'cpo'
>>> f.tell()
1842
>>> f.tell()
1869
>>> f.seek(1800)
>>> f.tell()
1841
>>> f.read(3)
fff
'fff'

So, it's probably the right thing to do… and probably safe to do. Probably.

[matsjoyce]

Which version of python is that?
I get

Python 3.4.1 (default, May 19 2014, 17:23:49) 
[GCC 4.9.0 20140507 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> f=sys.stdin
>>> f.read(3)
asdfghjkl
'asd'
>>> f.tell()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: underlying stream is not seekable

Python 2.7.6 (default, Feb 26 2014, 12:07:17) 
[GCC 4.8.2 20140206 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> f=sys.stdin
>>> f.read(3)
asdfghjkl
'asd'
>>> f.tell()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 29] Illegal seek

[matsjoyce]

The position is only set if it is non negative

[mmckerns]

Sorry, I clipped that off…

Python 2.7.6 (default, Nov 12 2013, 13:26:39) 
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] on darwin
Type "help", "copyright", "credits" or "license" for more information.

[matsjoyce]

Maybe its a mac - linux difference?

[mmckerns]

Probably not. More likely a new "feature" since yours is Feb 2014 and mine is a Nov 2013 build

[mmckerns]

Nope. I'm wrong, you are right. http://www.gossamer-threads.com/lists/python/python/99983
It seems to be an old issue.

[mmckerns]

It seems like a fine thing to do, as if you are pickling within a single session, the stdin queue will be preserved. If you dump_session (or go off-processor), I'd assume the queue getting cleaned out might be ok.

>>> f.read(3)
asd;vkadpsvjkapjvkpqejvpqejvqerv
'asd'
>>> 
>>> _f = dill.copy(f)
>>> _f
<open file '<stdin>', mode 'r' at 0x100e7a0c0>
>>> 
>>> _f.read(3)
';vk'
>>> f.read(3)
'adp'

This is indeed what happens by not setting the position.

>>> dill.dump_session('foo.pkl')
>>> 
dude@hilbert>$ python
Python 2.7.6 (default, Nov 12 2013, 13:26:39) 
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import dill
>>> dill.load_session('foo.pkl')
>>> f
<open file '<stdin>', mode 'r' at 0x1076900c0>
>>> f.read(3)
ddd
'ddd'

But it also seems like that would be the case for setting the position as well. Since, the queue seems to be maintained as long as you don't seek.

[mmckerns]

@matsjoyce: I'm sending you an email about a new release, and some etc.