[Python-Dev] Does Python need a file locking module (slightly higher level)?

[Barry Warsaw]

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On Oct 22, 2007, at 11:30 PM, skip at pobox.com wrote:

> It's not clear that any of these implementations is going to be  
> perfect.
> Maybe none ever will be.

I would agree with this.  You write a program and know you need to  
implement some kind of resource locking, so you start looking for  
some OTS solution.  But then you realize that your application needs  
somewhat different semantics or needs to work in platforms or  
environments that the OTS code doesn't handle.  Just a few days ago,  
I was looking at some locking code that needed to work across  
multiple invocations of a script on multiple machines, and the only  
thing they shared was a PostgreSQL connection, so we ended up wanting  
to use its advisory locks.

> In his reply Jean-Paul made this comment:
>
>     It might be nice to have something like that in the standard  
> library,
>     but it's very simple once you know what to do.
>
> I'm not so sure about the "very simple" part, especially if you aren't
> familiar with all the ins and outs of the different platforms.

I'd totally agree with this.  Locking seems simple, but it's got some  
really tricky aspects that need to be coded just right or you'll be  
in a world of hurt.  Mailman's LockFile.py (which you're right is  
*nix only) is stable now, but has had some really subtle bugs in the  
past.

> The fact
> that the first three bits of code I was referred to were  
> implemented by
> three significant Python tools/platforms and that all are different  
> in some
> significant ways suggests that there is some both an underlying  
> need for a
> file locking mechanism but with a lack of consensus about the best  
> way to
> implement the mother-of-all-file-locking schemes for Python.  Maybe  
> the best
> place for this is in the distribution.  PEP?

I don't think any one solution will work for everybody.  I'm not even  
sure we can define a common API a la the DBAPI, but if something were  
to make it into the standard distribution, that's the direction I'd  
go in.  Then we can provide various implementations that support the  
LockingAPI under various environments, constraints, and platforms.   
If we wanted to distribute them in the stdlib, we could put them all  
in a package and let the user decide which features they need.

I'm still planning on de-Mailman-ifying LockFile.py sometime soon.

- -Barry

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