[BangPypers] calling instance of the other class.

Anand Balachandran Pillai abpillai at gmail.com
Wed Jun 24 09:15:52 CEST 2009

On Wed, Jun 24, 2009 at 7:42 AM, Jeff Rush <jeff at taupro.com> wrote:

> learningpython wrote:
> > Hi Jeff,
> > Can i ask you a related question. Long shot ..
> > but since you have good idea of area i am working ( which is
> communications,
> > sending, decoding, encoding) data over a serial interface.
> > I have a problem, i am facing.. Like in ASN format..
> > I have a Data Unit ( am defining a class for each )
> >
> > 1. msg type  ( 8 bits )
> > 2. Identity(8 bits)
> > 3. ADESC ( 8 bits)
> > 4. Address ( Now this depends on value of ADESC )
> > so the other elements, i am allocating the memory, but how can i allocate
> > Address, the number of fields ( i mean size) depends on the ASESC value
> so
> > it could be any value .. how can i say reserve memory .
> >
> > The only idea i get is to have a function within the class of this
> > definition where, i read the ADESC and then allocate value in address.
> Any
> > other good ideas, please can you let me know. I really appreciate your
> > links.
> The important module for bit manipulation in Python in the struct
> module.  If you haven't come across it before, you should read the docs.
>  It is part of the Python standard library.
>  http://docs.python.org/library/struct.html

 I was just scanning the replies wondering why someone has not
 mentioned "struct" so far!

 "struct" is very useful to pack data structures in/out of the wire.
 In fact if you want to convert your data structure to a bytestream, you
 should first investigate the possiblity of using "struct" before writing
 custom code or searching other solutions.

> Once you've done that, check out this cool module that does much of what
> you are trying to do.  Fair warning though, it uses very advanced Python
> coding including metaclasses.  You'll learn a lot by carefully figuring
> out how it does what it does.  For example it avoids the need for a
> self.order list to sequence the fields of a class definition in a rather
> clever fashion.
>  http://code.activestate.com/recipes/498149/

 This seems like a very useful recipe. Often one requres something
 which can handle nested objects which struct cannot do, since it wants
 things to be presented to it as basic data types.

> I'm probably going to do a source code walkthrough of this module with
> the Dallas usergroup this Saturday because of its interesting parts.
> -Jeff
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