Error Remains After Running 2to3.py on a Module

Question

I used the 2to3.py script to convert several of my files to Python 3 a while back. I believe I need to run all fixers, so my command included

-f all -f buffer -f idioms -f set_literal -f ws_comma -w

I tried to run my converted code with Python 3, but got an error

[Errno 22] Invalid argument

on the line

stream.seek(-2,1)

stream is a StringIO object that is being used to parse a file. Is this a known difference in Python 2 and 3, so should I use different methods/syntax? Or is the issue in the 2to3 conversion - perhaps I didn't run the tool correctly. (I mean to run as many fixers as possible)

Answer 1

I don't know for sure, but I'm guessing this is a casualty of the new Unicode handling in 3.x:

In [3]: file_ = open('/etc/services', 'r')

In [4]: file_.readline()
Out[4]: '# Network services, Internet style\n'

In [5]: file_.readline()
Out[5]: '#\n'

In [6]: file_.readline()
Out[6]: '# Note that it is presently the policy of IANA to assign a single well-known\n'

In [7]: file_.seek(-2, 1)
---------------------------------------------------------------------------
UnsupportedOperation                      Traceback (most recent call last)
<ipython-input-7-6122ef700637> in <module>()
----> 1 file_.seek(-2, 1)

UnsupportedOperation: can't do nonzero cur-relative seeks

However, you can use binary I/O to do this:

In [9]: file_ = open('/etc/services', 'rb')

In [10]: file_.readline()
Out[10]: b'# Network services, Internet style\n'

In [11]: file_.readline()
Out[11]: b'#\n'

In [12]: file_.readline()
Out[12]: b'# Note that it is presently the policy of IANA to assign a single well-known\n'

In [13]: file_.seek(-2, 1)
Out[13]: 112

BTW, 3to2 is more effective than 2to3 if you want to maintain a dual codebase for a while. Also, many people (including me) are having luck maintaining a single codebase that runs on 2.x and 3.x, rather than using 2to3 or 3to2.

Here's a link to a presentation I gave about writing code to run on 2.x and 3.x: http://stromberg.dnsalias.org/~dstromberg/Intro-to-Python/

PS: Analogous to StringIO, is BytesIO:

In [17]: file_ = io.BytesIO(b'abc def\nghi jkl\nmno pqr\n')

In [18]: file_.readline()
Out[18]: b'abc def\n'

In [19]: file_.seek(-2, 1)
Out[19]: 6