On 07/01/2010 01:28 PM, james bardin wrote:
On Thu, Jul 1, 2010 at 1:00 PM, Nikolaus Rath<nikol...@rath.org> wrote:
Generally in python, the only objects you can share "without explicit
locking" are single instances of core data types - basically lists and
dicts.
As well as third-party modules that have been designed to be threadsafe.
*Modules* can be thread-safe, but what's an example of a module that
advertises it's classes as thread-safe? Any class that does that would
need to self-lock on all non-atomic operations. It's normally just
easier to expect locking to be handled outside of the class.
In most cases you are probably right. But I think there are also good
cases where locking is better done in the class itself. The class does
not need to self-lock all non-atomic operations, only those that
actually operate on instance (or global) variables. And even when
locking is required, the method is able to lock just the one variable it
is working with rather than the entire method.
Example: I am working with a class that uploads data to Amazon S3
(basically an online storage service with a simple HTTP API). The class
provides methods like put_from_fh(key, fh) and get_to_fh(key, fh) which
are designed to be threadsafe. When called, they first compress and
encrypt the data, then they briefly obtain a lock to get a HTTP
connection from a pool, release the lock and upload the data.
The methods have to be multithreaded because they provide a file system
backend (and it would be rather annoying if you would have to wait for
you 100 MB write into file1 to complete before you can read 10 bytes
from file2).
There are of course other possible implementations, like giving every
thread its own S3 storage instance or managing the storage classes in a
pool (instead of the HTTP connections), but I consider those to be less
elegant. The S3 storage class has semantics like a dict (only that the
amount of stored data is larger and stored elsewhere), so I would
consider it quite awkward if I had to bother with locking or pooling
when using it.
Best,
-Nikolaus
Btw, I am about to write a class that provides the same functionality
over SFTP, thus my initial question.
--
»Time flies like an arrow, fruit flies like a Banana.«
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