Question

I have read this question but it isn’t clear to me. I defined my class like this:

from sqlite3 import Connection, Cursor, Row, connect

class myclass(object):
    def __init__(self,number):
        co = connect('C:\\mydatabase.sqlite')
        co.row_factory = Row
        with connection:            
            cu = connection.cursor()
            sql = '''SELECT * FROM mytable WHERE Number= {n} LIMIT 1'''.format(n = number)
            cu.execute(sql)
            for i in cu:
                self.classattribute1 = i['Field1']
                self.classattribute2 = i['Field2']
                etc.

Now this works fine until I want to add a third attribute to my class like:

self.classattribute3 = self.classattribute1 + self.classattribute2

AttributeError: 'myclass' object has no attribute 'classattribute1'

This won’t work if the SELECT statement didn’t return anything, if the number isn’t in the database.

Now what I would like to do when I call an instance of myclass like:

myclassinstance1 = myclass(100)

I would like to write something like:

if cu.fetchone() == None:
    #code to exit the method __init__ and to delete my class instance here

I don’t know how to exit and delete the instance I called from inside myclass. I need to delete theses instances because I don’t want to use empty class instances.

Thanks for reading.

Answer 1

Just create a factory function that will either return a new instance or None if if couldn’t be loaded:

class MyClass(object):
    def __init__(self, attribute1, attribute2, ...):
        self.attribute1 = attribute1
        self.attribute2 = attribute2
        # ...

    @staticmethod
    def load_from_db(number):
        # set up and query database
        record = cursor.fetchone()
        if record == None:
            return None
        else:
            return MyClass(record['Field1'], record['Field2'], ...)

Then load MyClass object from a database with:

my_obj = MyClass.load_from_db(number)

You can’t delete an object in Python (from __init__ or anywhere), you can only delete a single reference to an object from a scope that contains this reference. (E.g. a scope calling MyClass(), like the load_from_db() function in the code above.)