Question

I have a class Foo which contains a datamember of type Bar. I can’t make a generalized, “default” Bar.__init__() – the Bar object is passed into the Foo.__init__() method.

How do I tell Python that I want a datamember of this type?

class Foo:

# These are the other things I've tried, with their errors    
    myBar         # NameError: name 'myBar' is not defined
    Bar myBar     # Java style: this is invalid Python syntax.
    myBar = None  #Assign "None", assign the real value in __init__. Doesn't work 
#####

    myBar = Bar(0,0,0) # Pass in "default" values. 
    def __init__(self, theBar):
        self.myBar = theBar

    def getBar(self):
        return self.myBar

This works, when I pass in the “default” values as shown. However, when I call getBar, I do not get back the one I passed in in the Foo.__init__() function – I get the “default” values.

b = Bar(1,2,3)
f = Foo(b)
print f.getBar().a, f.getBar().b, f.getBar().c

This spits out 0 0 0, not 1 2 3, like I’m expecting.

If I don’t bother declaring the myBar variable, I get errors in the getBar(self): method (Foo instance has no attribute 'myBar').

What’s the correct way to use a custom datamember in my object?

Answer 1

You don’t need to tell Python you are going to add a certain data member – just add it. Python is more dynamic than e.g. Java in this regard.

If bar instances are essentially immutable (meaning they are not changed in practice), you can give the default instance as default value of the __init__() parameter:

class Foo:
    def __init__(self, the_bar=Bar(0,0,0)):
        self.my_bar = the_bar

All Foo instances uisng the default value will share a single Bar instance. If the Bar instance might be changed, this is probably not what you want, and you should use this idiom in this case:

class Foo:
    def __init__(self, the_bar=None):
        if the_bar is None:
            the_bar = Bar(0,0,0)
        self.my_bar = the_bar

Note that you shouldn’t usually write getters and setters in Python. They are just unnecessary boilerplate code slowing down your application. Since Python supports properties, you also don’t need them to be future-proof.