[Zope] threading and External Methods

[Jim Fulton]

Brian Hooper wrote:
> 
> Hi there -
> 
> Real simple question: Do I need to worry about threading when coding
> External Methods (in Zope 2)? 

Certainly.  Here are basic rules:

  - You don't have to worry about state of
    persistent objects (e.g. self). Persistent
    objects are inherently thread safe, since each
    thread has it's own copy.

  - You don't have to worry about state of immutable
    objects, as they can't be modified by multiple
    threads.

  - You do need to worry about global variables
    (e.g. module variables and shared instance
    variables defined in class statements) and
    mutable default arguments to functions and 
    methods.

> If I do have something that is not safe
> (for example, I want to os.system nslookup to check MX records)

Hm. Why is this not safe? This is definately pretty heavy.

> do I have to explicitly protect it somehow?

Yes.  This can be done with a simple lock object, or one of the
various modules distributed with Python.

Jim

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