[Python 2.x] Pickling a datetime.tzinfo subclass instance?

[Gabriel Genellina]

En Mon, 08 Dec 2008 12:34:03 -0200, Cong Ma <cma at mail.bnu.edu.cn> escribió:

> I'm writing a program that pickles an instance of a custom subclass of
> datetime.tzinfo. I followed the guides given in the Library Reference  
> (version
> 2.5.2, chapter 5.1.6), which contain the note:
>
> "Special requirement for pickling: A tzinfo subclass must have an  
> __init__
> method that can be called with no arguments, else it can be pickled but  
> possibly
> not unpickled again. This is a technical requirement that may be relaxed  
> in the
> future."
>
> I tried this with an example "FixedOffset" subclass instance given in the
> Example section in the manual. It indeed failed to unpickle. To work  
> around
> this, I found two possible solutions:
> 1. Modify the __init__ method so that it takes optional arguments with  
> default
> values;

Doing that still works with 2.6 and 3.0

> 2. Implement the __getinitargs__ method so that it does the opposite of
> __init__: returning a tuple from the instance's internal state that can  
> be used
> to re-initialize an instance, retaining the old value.

In fact, it doesn't matter *what* it returns, as far as they're valid
arguments to __init__

> My questions:
> 1. Is the "technical limitation" fixed in version 2.6 or 3.0? I can't  
> check it
> for myself now... Python.org seems down and I can't find the docs.

No, they behave the same (odd) way.

> 2. To stick with version 2.5, which of the above 2 methods is better?  
> Both seems
>  to unpickle to the correct result, but are there subtle side-effects?  
> Or there
> are better solutions?

I'd use method 1, just because the __getinitargs__ are useless.

-- 
Gabriel Genellina