How to perform common post-initialization tasks in inherited Python classes?

Question

The initialization process of group of classes that share a common parent can be divided into three parts: common part1, class-specific part, common part2. Currently the first two parts are called from the __init__ function of each child class, but the second common part has to be called separately For example:

class BaseClass:
    def __init__(self):
 print 'base __init__'
 self.common1()

    def common1(self):
 print 'common 1'

    def finalizeInitialization(self):
 print 'finalizeInitialization [common2]'


class Subclass1(BaseClass):
    def __init__(self):
 BaseClass.__init__(self)
     self.specific()

    def specific(self):
 print 'specific'


if __name__ == '__main__':
    s = Subclass1() #Don't forget to finalize the initialization
    s.finalizeInitialization()  # now the object is fully initialized

Is there a way to not to have to call finalizeInitialization()?

EDIT one can transfer the call to finalizeInitialization() into Subclass1's __init__ (as in S.Lott's answer). This makes the life easier, but still one has to remember to complete the initialization, this time inside the "constructor". Either way there is no way to enforce full initialization, which is what I'm looking for.

Answer 1

Version 1 - delegate everything.

class Subclass1(BaseClass):
    def __init__(self):
        super( Subclass1, self ).__init__()
        self.specific()
        super( Subclass1, self ).finalizeInitialization()

Version 2 - delegate just one step

class BaseClass:
    def __init__(self):
        print 'base __init__'
        self.common1()
        self.specific()
        self.finalizeInitialization()

    def common1(self):
        print 'common 1'

    def finalizeInitialization(self):
        print 'finalizeInitialization [common2]'

    def specific( self ):
        # two choices:
        # if this is "abstract": raise an exception
        # if this is "concrete": pass

Answer 2

What's wrong with calling finalInitilazation from the Subclass's init?

   class BaseClass:
        def __init__(self):
            print 'base __init__'
            self.common1()

        def common1(self):
            print 'common 1'

        def finalizeInitialization(self):
            print 'finalizeInitialization [common2]'


    class Subclass1(BaseClass):
        def __init__(self):
            BaseClass.__init__(self)
            self.specific()
            BaseClass.finalizeInitialization(self)

        def specific(self):
            print 'specific'


    if __name__ == '__main__':
        s = Subclass1() #Don't forget to finalize the initialization
        s.finalizeInitialization()  # now the object is fully initialized

Answer 3

If you have to call multiple methods in a specific order it typically means that the design has problems to begin with (it's leaking implementation detail). So I would try to work from that end.

On the other hand if people derive from the class and have to modify the initialisation they should be aware of the implications - it is not something you would want to have in your normal API. Alternatively you could be defensive about the final initialization and check that it has been called in methods that depend on it (to call it or raise an exception if not).

Answer 4

Template Method Design Pattern to the rescue:

class BaseClass:
    def __init__(self, specifics=None):
        print 'base __init__'
        self.common1()
        if specifics is not None:
            specifics()
        self.finalizeInitialization()

    def common1(self):
        print 'common 1'

    def finalizeInitialization(self):
        print 'finalizeInitialization [common2]'


class Subclass1(BaseClass):
    def __init__(self):
        BaseClass.__init__(self, self.specific)

    def specific(self):
        print 'specific'

Answer 5

Similar to S. Lott's approach, except there's no way (short of overriding __init__) for the derived classes to override (or even call) the common methods:

class BaseClass:
    def __init__(self):
        def common():
            print "common initialization..."

        def final():
            print "common finalization..."

        common()
        self.specific()
        final()

    def final_init(self):
        print "BaseClass.final_init"


class Subclass1(BaseClass):

    def specific(self):
        print "Subclass1.specific"

You might want to provide a default implementation of specific in BaseClass if it's not okay to raise an AttributeError when you create an instance of any subclass that doesn't provide its own implementation.