Python: Getting baseclass values from derived clas

Question

Hope this is clear enough:

class myParent():
    def __init__( self ):
        self.parentNumber = 5

class Child( myParent ):
    def __init__( self ):
        self.childNumber = 4

    def multiplyNumbers( self ):
        print myParent.parentNumber * self.childNumber

p = Child()
p.multiplyNumbers()

I wish to set the parentNumber individually, and then reach that number via the child class, and in this case use it for some multiplication.

I'm new to the OOP area so any general pointers on inheritance is welcome as well!

More info: I'm designing a project management solution for vfx-based projects, and am playing with classes and inheritance to see how they can help me the most.

Right now, I've got the top class, Project, and a derived class, Shot. Shot has a self.length variable with the length of the specific shot. It's also got a getLengthInSeconds() method that uses self.length along with the Project.fps to determine the length in seconds. Project has a setFps() method in which the fps is set after an instance of the class is created.

I'm kind of used to variables being prefixed with self. and have not experimented much with classes using the more "global" variables without self. . If I make everything global, no self., I can use Project.fps without hassle, but I'm getting a "bad programming practice" warning in my neckhair. Perhaps there is a better, more neat, way?

Edit:

After some reading, super() seems kinda dangerous, and a bit more than I need I think. I am mainly having single-inheritance classes and aren't even sure how to make use of diamond hierarchies.. Is there a safer way to access superclass variables and methods that doesn't include super()?

Edit:

Allright, see if this makes sense or if I'm thinking about it all wrong.

I'm looking at classes and inheritance as groups and children. A child knows it's parent and all it's values. A child to another parent knows That parents values. What I'm trying to accomplish is having all shots created be part of a project. And right now, I'm creating Shot() instances from within Project() class, adding the instances to a list of shots which is then maintained within the Project() instance.

i.e.

class myParent( object ):
    def __init__( self ):
        self.parent_id = ''
        self.children = []

    def createChild( self, name ):
        self.children.append( myChild( name ) )

    def getChildren( self ):
        return self.children

    def setParentId( self, id ):
        self.parentId = id

class myChild( myParent ):
    def __init__( self, id ):
        super(myChild, self).__init__()
        self.id = id

    def getParentId( self ):
        return self.parent_id

p = myParent()
p.setParentId( 'parent01' )
p.createChild( 'child01' )
print p.getChildren()[0].getParentId()

I can sort of see the mis-steps in logic here, but no real way around it.. Seems like every child is getting a new instance of the parent this way, where parent_id is always an empty string.

Answer 1

You just need to tell Child to run the initialization from myParent. After that, the current instance (self) has all the instance attributes an instance of myParent would have ("is a"), so you can use self.parentNumber. You do so by putting super(Child, self).__init__() into Child.__init__ (ideally, on the very first line) - myParent.__init__(self) would work, but it can be wrong (in subtle way) when the base class changes or in case of multiple inheritance. You can drop all the arguments to super if you're using Python 3.

Also note that

class C(object):
    x = ...

is very different from

class C(object):
    def __init__(self):
        self.x = ...

The former creates a class variable, which is shared by all instances of C. The latter create an instance variable, each instance of C has it's own. (Likewise, global variables aren't bound to instances - but I assume you were talking about the above?)

Answer 2

class myParent( object ):
    def __init__( self ):
        self.parentNumber = 5

class Child( myParent ):
    def __init__( self ):
        myParent.__init__( self )
        self.childNumber = 4

    def multiplyNumbers( self ):
        print self.parentNumber * self.childNumber

p = Child()
p.multiplyNumbers()

You can usually manage just fine without super when you don't have multiple inheritance or other border cases, although if the base class changes, you will need to remember to change the parent class name from init (and any other method that refers to myParent explicitly).

If your parent's __init__ takes parameters, you need to pass them on from the child's __init__.

class myParent( object ):
    def __init__( self, customParam ):
        self.parentNumber = 5
        self.customParam = customParam

class Child( myParent ):
    def __init__( self, customParam ):
        myParent.__init__( self, customParam )
        self.childNumber = 4

If you dislike the repetition of customParam in all child classes, there's an alternative OO pattern called two phase construction, which works like this:

class myParent( object ):
    def customInit( self, customParam ):
        self.parentNumber = 5
        self.customParam = customParam

class Child( myParent ):
    def __init__( self, customParam ):
        self.childNumber = 4

p = Child()
p.customInit(10)
p.multiplyNumbers()

In this pattern, you don't need to repeat any of the parent's parameters, or even call the parent's __init__ in the child, but the downside is that you will need to remember to always call the secondary constructor when creating objects, or your object will be left partially uninitialized.

UPDATE (to answer the updated question):

You seem to be mixing two unrelated concepts of parenthood here. Inheritance is about type hierarchy, and you seem to be after an ownership hierarchy (is-a versus has-a).

I would structure your updated code like this:

class myParent( object ):
    def __init__( self, parentId ):
        self.id = parentId
        self.children = []

    def createChild( self, name ):
        self.children.append( myChild( name, self ) )

    def getChildren( self ):
        return self.children

class myChild( object ):
    def __init__( self, childId, parent ):
        self.id = childId
        self.parent = parent

    def getParentId( self ):
        return self.parent.id

p = myParent( 'parent01' )
p.createChild( 'child01' )
print p.getChildren()[0].getParentId()