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

Steven D'Aprano steve at pearwood.info
Tue Feb 12 02:34:28 CET 2013

On 12/02/13 10:16, Alan Gauld 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

Strictly speaking, a class method is just a method which takes as its
first argument the class itself, not the instance. What it does with
that is completely open.

The usual thing is to use class methods for alternative constructors,
e.g. Decimal.fromfloat.

If the class keeps a list of all instances, then the class method
could walk the list and operate on each instance in turn. (But why
would you do that?)

If the class method modifies the class itself, then it could indirectly
have an effect on each instance.

Although class methods could do anything, it is hard to think of
actually useful things for them to do apart from being used as a

>> 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.

Just a minute, I think that is completely wrong. A Book is not a set,
so how can you have subset of it?

What is a subset of "Pride and Prejudice"? Perhaps chapter 5.

There are more problems with this idea that you query the Book to get
a list of books by some author. Suppose you did this:

prpr = Book("Pride and Prejudice", "Jane Austin")

Now *each and every* book is responsible for tracking all the other
books by the same author. This is a lousy design. Even worse:

prpr.list_by_author("Jane Austin")

since now books are responsible for tracking ALL books by ALL authors,
since the caller could say:

prpr.list_by_author("Leo Tolstoy")

Of course, books should know their own author, not the authors of other
books, but authors should know all their own books:

author = prpr.author  # --> Author("Jane Austin")

author.get_books()  # --> return a list of books by this author

This is an argument for making Authors a class, with behaviour, rather
than just a string.

>> @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

First off, by convention the first argument to a class method should be
called "cls", not "self".

Secondly, here you are relying on a mysterious global "config", which
points to a bookfile. What does this have to do with a book?

- Does a nail keep track of the packet it came from?

- Why should a book keep track of the catalog it was listed in?

This should be a top level function, not a Book method.

The rest of the method's design is also poor. You have already read
the file once, to get the initial set of books. So why read the file
again, every time you want to get some piece of information.

Big databases, capable of holding billions of pieces of data, have
to use disk-based storage because you can't keep that much data in
memory at once. For anything smaller, you should only read and write
to disk for persistence, everything else should use in-memory data
structures. In this case, that means a dict.

>> return books # return list of books

Really? Thank goodness for the comment, I wouldn't have understood
that line of code otherwise!



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