ultra newbie question (don't laugh)

Larry Bates larry.bates at websafe.com
Tue Sep 26 16:38:03 CEST 2006


John Salerno wrote:
> Ok, I've decided to make a little project for myself which involves
> storing employee information in an XML file. I'm doing this partly to
> experiment with working with XML. The blocks in the file will look
> something like this:
> 
> <researcher id="salerjo01">
>   <first_name>John</first_name>
>   <last_name>Salerno</last_name>
>   <birth_country>United States</birth_country>
>   <birth_state>Texas</birth_state>
>   <birth_city>Houston</birth_city>
>   #etc.......
> <researcher>
> 
> I also plan to make a GUI frontend with wxPython for entering the
> records. This will be fairly easy, but not as fun as writing the logic.
> For now I've decided to focus just on writing the logic, and not worry
> about the GUI yet.
> 
> So this is what I came up with so far, then I sat staring at the screen
> wondering how to proceed:
> 
> class LabXMLWriter(object):
> 
>     def write_name(self, first, last, given=''):
> 
> Suddenly I realized I just have no idea how to start thinking about what
> I need to do. My first instinct was to use a class, as above, but then I
> wondered if that was even necessary, since all I need to do is get
> information from a user and write it to a file. Do I really need that
> information stored in an object?
> 
> Then I wondered if I needed an __init__ method, and what could go in it?
> Or should I just make separate methods for each bit of information to
> write to the file (i.e., name, birth location, address, phone number,
> etc.). I thought maybe I could create the ID in the __init__ method
> (salerjo01), but to do that I need the name first. I could do this:
> 
> def __init__(self, first_name, last_name, given_name=''):
>     # code to initialize name and create ID
> 
> But then I wondered if this detracts from the work that the class
> methods would do later.
> 
> So you see, what I'm asking for is very basic help, sort of along the
> lines of "what things do I need to consider before I even begin this?"
> Is OOP necessary here? Would utility functions work just as well for
> simply writing the information to a file?
> 
> Perhaps I should just take some kind of programming intro class! I read
> all these Python books, but when it comes time to write something
> non-trivial, I get stuck almost immediately with all the possibilities.
> 
> Thanks,
> John
On reason to use OOP is to insulate your main code from things like storage
backends or future changes you may decide to make.  What if you change your mind
and want to store the data into something other than XML? If you move all your
storage code into an object and have it do the writing/reading then you can
easily replace that object without disrupting your main program.  You could even
extend your program and allow someoneto store in CSV, database, or other storage
by adding additional storage objects.  I had a project where I was reading data
from CSV file.  Later in the implementation the client decided that the file
that was to be processed would be a .ZIP file instead of a plain-text CSV.
Because I had implemented a class to do the reading of all data from the CSV
file that had implemented an iterator, I was able to insert code to open .ZIP
file (using zipfile module), open a file inside the .ZIP and read using CSV
module from the zipped file without ever actually unzipping the file. I didn't
change any top level code and the change took a just a few minutes to implement.
 There was little chance of creating unintended problems and I was able to
easily support both the old unzipped CSV filesAND the new zipped files.  If I
had not isolated that code into a class, the changes would have been a LOT more
difficult.

It can also be nice to provide for future expansion by supporting additional
keyword arguments.  Something like:

def __init__(self, first_name, last_name, **kwargs):

Then process the kwargs for additional fields.  This way you can painlessly add
a field to your code by just specifying an additional keyword argument. If the
application is as simple as you describe it may not be worth the additional
effort.  Once you have done one of these it will become second nature to think
of creating/using objects in places where you want to  provide for maximum
flexibility and code isolation.

At a minimum you should take a look at the elementtree module for handling your
XML.  It is downloadable for Python < 2.5 and is part of the standard library
starting with version 2.5.

Hope information helps at least a little.

-Larry Bates



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