[Tutor] associating two objects without ORM and processing a text file

neubyr neubyr at gmail.com
Mon Feb 11 23:49:39 CET 2013

On Mon, Feb 11, 2013 at 12:36 PM, Dave Angel <davea at davea.name> wrote:

> On 02/11/2013 01:19 PM, Alan Gauld wrote:
>> On 11/02/13 05:14, neubyr wrote:
>>> <snip>
>>  * How do I associate/relate Book and Author classes so that it will help
>>> me in getting information like 'get list of books written by an author'?
>>> Data attribute?
>> I woudn't have a separate Author class but, if you must, something like:
>> class Book:
>>    def __init__(self, theAuthor,theTitle):
>>        self.Author = theAuthor
>>        self.title = theTitle
>> class Author:
>>    def __init__(self,aName):
>>       self.name = aName
>> myBook = Book(Author('Jane Austin'), 'Pride & Prejudice')
> Big problem with that;  then there may be multiple instances of Author,
> representing the same Author.  Instead, there needs to be a factory
> function which reuses the same Author objects if an additional book with
> the same author is encountered.  Without such an approach, one might as
> well stick with strings, which is what we each recommended.
> --
> DaveA
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Thank you for suggestions - Mitya, Dave and Alan.

I am doing it as a learning exercise and it's not a class assignment. I
have little experience with Ruby and now trying to learn Python. It's not
going to be any production application and I haven't thought about
concurrency problems yet.

Based on suggestions, following is a code snippet that I have right now. I
agree that there isn't any need of separate Author object right now, so I
may remove it as well. I have created a class method 'list_by_author' to
return list of books. I am not sure if class method is right approach to
implement 'list_by_author' function as a class method is typically used as
an alternative constructor. Here I am returning list of objects and not
just an object.

Any suggestions for improving this code will be really useful.

class Book(object):
  def __init__(self,name,author,genre,pubdate):
    self.name     = name
    self.author   = author
    self.genre    = genre
    self.pubdate  = pubdate

  # TODO: use csv module
  # write/add method
  def add(self,uname,kname):
    """ Write book info to a file """

  def list_by_author(self,author):
    """ Return list of books of an author """
    bookfile = config.bookfile
    books = [] # empty list - used as list of Books
    # TODO: improve regex
    regex = re.compile(author)
    with open (bookfile,'r') as f:
      for line in f:
        if regex.findall(line):
          # error prone - if name contains comma
          l = line.split(',')
          # create book object and append it to a list
          book = self(*l)
    return books # return list of books

class Author(object):
  def __init__(self,name):
    self.name = name

  def books(self):
    """ Get list of books """
    books = Book.list_by_author(self.name)
    return books

- N
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