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

neubyr neubyr at gmail.com
Tue Feb 12 01:55:36 CET 2013

On Mon, Feb 11, 2013 at 5:16 PM, Alan Gauld <alan.gauld at btinternet.com>wrote:

> On 11/02/13 22:49, neubyr wrote:
>  is right approach to implement 'list_by_author' function as a class
>> method is typically used as an alternative constructor.
> Not at all that is only one use case for class methods.
> A class method is *any* method that operates on the whole class of objects
> - i.e. all instances(potentially including those still to be created)
> Now I agree that in some lanmguages they tend to be limited to factory
> metjods(eg Objective C implicitly goes that way) but in others  like
> Smalltalk search methods and persistence etc are often found as class
> methods. In Python, because of it's hybrid nature, these are often
> "demoted" to global scope functions.
>  Here I am
>> returning list of objects and not just an object.
> Which is to say a subset of the class Book.
> Therefore quite reasonably a class method.
>     @classmethod
>>    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)
>>            books.append(book)
>>      return books # return list of books
> I'd probably, for a small dataset, load a list of books into the class at
> startup and save it at termination. Which makes the search code easier and
> potentially can be data driven so you pass the filter attribute as a
> parameter. You can then use getattr() to find the value:
> def findBooks(cls, param, value):
>     return [book for book in books if getattr(book, param) == value]
> You could make it more flexible still by defining the filter as a function
> and passing a lambda:
> def findBooks(cls, filterExp):
>     return [book for book in books if filterExp(book)]
> Usage example:
> DickensBooks = Book.findBook(lambda b: 'Dickens' in b.author)
> Of course if you have a huge dataset then that won't work - which brings
> us back to a database :-)
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/

That's really helpful Alan. Thank you for your inputs on class methods and
how can I modify my existing find/list method.

I am not using any database/ORM as I am trying to learn basic search/filter
operations on file and enumerator objects. Also, ORMs generally add magic
methods based on associations (at least with Ruby ActiveRecord: e.g.
has_many and belongs_to associations). I would like to do it 'manually'
before using any other library.

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