[Tutor] class initialization with a lot of parameters

[Alan Gauld]

"C.T. Matsumoto" <c.t.matsumoto at gmail.com> wrote

> First I've got a db class
>
> class DB(object):
>    """ This does all the database handling.
>    """

That's fine.

> Then I've got a class to filter a list of potential tables to be 
> compared.
> These tables need to be tested.

OK, Could that be a method of your database?

> class CompareTableList(DB):
>    """ Make a master list, from a source list of compare tables
>         that need to be compared.
>         If the tables in the list pass the tests make a CompareItem.
>         Store all the CompareItems in a compare_list.
>    """

I still think that from an OOP sense it would be m,ore logical to
have a Table cklass that knows hhow to compare itself to another table.

You are trying to build an object which is a function, thats rarely
a good idea. You wind up with the worst of procedural and OOP
worlds coming together.

> CompareItem is the class in question.
>
> CompareItem(object):
>    def __init__(self, ref_table, ref_dburi, ref_rows, test_table,
> test_dburi, test_rows, keys):
>        ...
>    ...
>
> This would be your suggested TestTable only it already has the pair and 
> is
> ready to be compared. Rather than a method compare is a class.

I suspect thats your problem. cComparing things is normally an
operation of a class not a class in its own right. A compare class
would need to inspect the internal details of at least 2 objects.
But objects should manage their own data not expose it to third parties.
So either you have to pass in all the object attributes you want to know
about - long parameter  lists or you pass in two objects and violate data
hiding within the comparison.

> Compare(CompareTableList):
>    """ Foreach CompareItem in the CompareTableList.compare_list. Workout
>         the differences and store difference in something that can format
>         a report.
>    """

Again this looks like a method of your database.

db.compare(tables)
     foreach table in tables:
           if table.compare(masterTable?):
                 storeDifferences()

or

db.compare(table_sets):
     for table1, table2 in table_sets:
           if table1.compare(table2)
                storeDifferences()

Or similar.

By trying to create objects which are functions instead of making
the function a method of the objects that they act on you are
complicating your code.

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/