Module baldy compiled to pyc?

[Steven D'Aprano]

On Thu, 27 Sep 2012 11:00:36 +0200, Laszlo Nagy wrote:

> Today I had a strange experience. I have copied some updated py files
> (modules) to a directory on a remote server, overwritting the old ones.
> The pyc files on the server were older. Many programs are importing
> these modules, and most of them are started as background jobs (from
> cron). They started to throw all kinds of errors. I checked the py
> files, and they did have class definitions inside. However when I tried
> to use them I got AttributeError exceptions. Telling that those things
> are not in the module.

It sounds to me like the timestamps on the .py files were messed up when 
you copied them, causing Python to prefer old, broken .pyc files instead 
of the newer .py files.


> I suspect that there were two (or more) programs starting at the same
> time, writting the same pyc file at the same time.

I suppose that's not impossible, although you would need to check the 
Python source code to be sure. But I would expect that each process 
writes the .pyc file atomically, so even if two processes both try to 
create the .pyc file, one will harmlessly overwrite the other's output. I 
would be surprised if it were possible for two processes to 
simultaneously write to the same file -- that would require the Python 
compiler to explicitly open the .pyc file in shared writable mode, and 
why would it do that?

Far more likely is a file system error. I recommend you run a disk check 
on the file server and see if it comes up with any errors.


-- 
Steven