Luke Lee

Software Engineer

Web + Desktop + Science

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Python versioning

Motivation

I recently spent a little time reviewing a Python bug report and determining if I was running with a fixed version of the Python interpreter. I think this is a useful exercise for someone who is overly curious and not a core developer of the language.

This post is a rundown of my thought process while trying to figure this out. Previously, I thought answering the question 'Does my Python interpreter have this bug fix?' would be easy, turns out it can be pretty involved. In fact, the process was complicated enough that I'm not 100 percent confident of my results. So, if you know how to answer this question in an easier way, or if I did something wrong feel free to drop me a comment.

Step 1: Is this a bug?

First, I stumbled onto a weird warning when trying to zip files in an older version of CPython. The warning looked like this:

DeprecationWarning: struct integer overflow masking is deprecated

I also noticed my zip file was corrupted. So, what's a curious developer to do? Obviously, researching this error was the only option. This research lead me to a bug report for CPython. Luckily, the issue was marked as closed and that revision 64688 contained the fix.

Awesome, the magic of open source and documentation saves the day, but wait I'm still seeing the error! Am I doing something wrong or does my version of the interpreter not have this fix?

Versioning rabbit hole

Have you ever wondered what the information printed when you run the Python interpreter really means? Let's break down the following for a typical Windows machine since it has a bit more information.

Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)] on
win 32
...

Ok, so I have Python 2.5.4, but does this include the commit in revision 64688? I could try using this 'release' number and dig through a bunch of release notes, but that seems difficult. I just want to know what specific subversion version number my interpreter is running to verify I have the fixed revision. [1]

Luckily, the raw version information, r254:67916, is available in the interpreter output. However, I'm still a bit confused, why are there two numbers?

Well, this version information actually comes from sys.subversion. So, r254 refers to the subversion branch or tag and 67916 is the version number.

Branch or tag?

I'm not aware of a good way to distinguish if the string r254 is a branch or a tag. So, we just have to manually look it up in the old repository. [2]

At this point we have jump into the Python subversion repository and find a branch or tag with the name r254. After some searching, you'll find r254 refers to this tag.

Tag traversal

Remember, the bug fix I was looking for was in revision 64688. I am running revision 67916, which is newer. However, that doesn't mean that my branch/tag has this commit. Again, this is because subversion revision numbers are linear and branching can make things complicated.

So, let's look into the revision I have a bit closer now that we have the correct tag. We could checkout this code locally and manually step through the commits. However, it's a bit faster and easier to use the web interface in this situation. Here's the quick process I used to dig into tag:

  1. Go to the exact location for the SVN r254 tag.
  2. Remember that my Python installation was using revision 67916. So, we need to see the state of this repository at this commit and tag. You can do this in the web interface by typing the revision in the sticky revision box and hitting set, or going here.
  3. Next, remember from the diff that the Lib/zipfile.py file was where the bug fix was committed. So, we need to look specifically at the commit logs for this file, which can be found here.

Verdict

It looks like the last commit to the Lib/zipfile.py file I was running was revision 60117 from January 19 2008. Therefore, it appears that I do NOT have the commit for the bug fix.

Review

My version of Python has a bug in the zipfile module, which results in this error:

DeprecationWarning: struct integer overflow masking is deprecated

I can tell that this was a bug and fixed a long time ago. Unfortunately, my actual revision doesn't have this fix. Now it seems safe to assume that is why I'm getting the original error, but I can verify this this too!

I can upgrade the Python installation to a later revision of the 2.5 series and verify that the bug is indeed fixed by the magic of open source! I'll leave that work as an exercise for the reader. Unfortunately, in my original production scenario upgrading the interpreter was not an option.

Side notes

[1] This is an old version of Python, which still used subversion as the RCS. Python has since moved active development to Mercurial.

[2] It's important to know the branch or tag because just knowing the revision 64688 is not enough. For example, if we are on revision 65689 that doesn't necessarily mean we have the previous revision 64688. This is because subversion revision numbers are linear when branching.

[3] All the links in this article reference Python version 2.5.4 since this was the version I was seeing the problems in.

Published: 06-02-2013 23:37:16

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