[Tutor] making onthefly attributes persistent

[Knacktus]

Am 13.12.2010 23:50, schrieb Jojo Mwebaze:
>
>
> On Mon, Dec 13, 2010 at 8:44 PM, Alan Gauld <alan.gauld at btinternet.com
> <mailto:alan.gauld at btinternet.com>> wrote:
>
>
>     "Jojo Mwebaze" <jojo.mwebaze at gmail.com
>     <mailto:jojo.mwebaze at gmail.com>> wrote
>
>         Assuming i have a class bank as below .
>
>         class bank(object):
>           def __init__(self, bal=0):
>               self.bal = bal
>           def deposit(self, amount):
>               self.bal+=amount
>               print self.bal
>
>         I define a method debit - which i add to the class onthefly
>
>         bank.debit = debit
>
>
>         #I can also add an attribute owner
>
>         myaccount.owner = 'jojo'
>
>
>         My problem is how to make the added attributes, 'owner' and 'debit'
>         persistent automatically
>
>
>     If that's your only problem with this approach congratulations!
>     How does your orther code know when/if these dynamic
>     operations/data exist so as to use them? If they just assume
>     they exist then why not just add them in the definition. Even as nulls?
>
>     While Python allows you to dynamically add features to classes/objects
>     its not something I would recommend unless you have a really good
>     reason - not least because you bring upon yourself all sorts of
>     problems!
>
>     If you are determined to do so you can make the objects persistent
>     using the approach I outline on my tutorial but adding a loop to cycle
>     over the contents of dir(). But you may find that recovering the
>     objects - especially if they have a mixed set of attribnutes - presents
>     even more problems...
>
>     IMHO This is a feature of python that should be considered unorthodox
>     and only to be used when no other approach will work!
>
>     HTH,
>
>
>
>
> Thanks Allan for the feedback, the idea is to write a method like
> store() on the object, that probably looks up all these changes and
> commits them into the database.

Assuming you're planning to use a database with a scheme (e.g. a 
relational DB) you need to define your tables in advance anyway. Even if 
you're using a schemeless DB, where do you track which data is in it? 
How do you build your queries dynamically? This can become very messy.
Overall, I think Alan's recommendation to avoid dynamic attribute 
creation is the one to follow.

>
> Please let me know where to find approach you propose in your tutorial.
> I read your tutorial when i was just initiated to python, a reference
> would be helpful to help me find the soln without hustle.
>
> Cheers
>
>
>
>     --
>     Alan Gauld
>     Author of the Learn to Program web site
>     http://www.alan-g.me.uk/
>
>
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