Metaclass to make all methods of a class thread-safe

Irmen de Jong irmen at -nospam-remove-this-xs4all.nl
Mon Sep 27 21:22:48 CEST 2004


Michele Simionato wrote:
> Irmen de Jong <irmen at -nospam-remove-this-xs4all.nl> wrote in message news:<415728eb$0$78753$e4fe514c at news.xs4all.nl>...
[...]
>>Is this the right approach? It seems to work fine. But I have
>>very little experience with metaclass programming, so I'd like
>>to hear some feedback.
>>
>>Thanks !!
>>
>>--Irmen de Jong.
> 
> 
> Well, it looks okay, but consider the following:
> 
> 1. you have a (metaclass) static method and a (metaclass) classmethod
>    which could be replaced by simple functions external to the metaclass;
>    this would make the code much easier to read and to understand; I don't
>    buy the argument that they should logically stay in the metaclass, it
>    is enough if they stay in the same module of the metaclass, not inside it;

Agreed, I was just modeling it after some example metaclass code that I found
on ASPN cookbook. The metaclass itself will only be a few lines then :-)

> 2. the metaclass will automagically wrap even methods of subclasses, without
>    you knowing it; consider using a naming convention (es. only methods
>    starting with "t_" are magically wrapped); if still
>    you want the magic, consider defining a name convention such as methods
>    starting with a given prefix are NOT magically wrapped; 

This is good advice. I started by not wrapping methods with '__' prefix...

> 4. built-in methods and all the objects which are non instances of FunctionType
>    will be not wrapped; you may want this or not;

Built-in methods such as?
About the other objects: I only cared about wrapping class methods.
Shouldn't I have / should I use something else than FunctionType?

There is one thing though; methods that you're accessing trough the
class's __dict__ (which is what the meta class is doing, right?)
are of type <function>, rathar than <instancemethod> which I expected:

 >>> class A:
...  def meth(self): pass
...
 >>> type(A.meth)
<type 'instancemethod'>
 >>> type(A.__dict__['meth'])
<type 'function'>
 >>>

Why is this?


> 5. consider using decorators to wrap the methods you want to be
>    thread safe: they a more esplicit and easier to understand solution;
>    also in this way you will avoid the (possible) issue of metaclass 
>    conflicts.

No decorators, nooo sir, it must work on Python 2.3 too :)

> 6. However, if you want to enhance a code which is already written
>    with a minimal change of the source code, the metaclass is the 
>    simplest solution indeed.

Thanks for your comments, Michele.

--Irmen



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