[Tutor] Object oriented design

[jamie hu]

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   20.12.2015, 01:25, "Alan Gauld" <alan.gauld at btinternet.com>:

     On 20/12/15 00:48, jamie hu wrote:

       ****trying to think/implement. I can create a given student object
       based on
       ****given firstname, lastname and grade. How do I find all objects
       matching
       ****particular criteria and return them to caller? Do I need to
       iterate/select
       ****through some list/database and create Student objects again?

     You need to store a referejce to each object in some kind of
     container - a list or dictionary for example, or maybe a full
     blown database.

       ****class Student():
       ***** def __init__(self,firstname,lastname,age):
       ***** * self.firstname = firstname
       ***** * self.lastname = lastname
       ***** * self.grade = grade

       ***** def set_grade(self,grade):
       ***** * self.grade = grade

       ***** @classmethod

       ***** # Find all Students with given lastname
       ***** def find_by_lastname():
       ***** * # How do I return all student objects that have same lastname?
       ***** * # Do I need to call init method again? I am confused here.
       ***** * pass

     You don't need to call init a second time but you do need to put
     your instances into a container as you create them. A common way
     to do this is to have a class attribute (ie not an instance one)
     called _instances or similar and have a line at the end
     of __init__() that does

     Student._instances.append(self)

     Your class method can then traverse the _instances collection
     checking each instance until it finds the desired object.

     If you have many objects, and especially if they will not
     all be instantiated at once you would use a database and
     in that case the class method would check the instances
     collection first to see if you already had the objectld
     in memory and, if not, instantiate it from the database.

     HTH

     --
     Alan G

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   Thanks Alan. I was thinking about making a list of objects and search
   through it, but wasn't sure if that was right way.
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   I wasn't sure instantiation needed when using database, but it seems like
   that's the way. So ORM libraries do this work (fetch data and return
   instantiated objects) behind the scenes??
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   --
   Thanks,
   Jamie
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