Question

class Foo(object):
    def __init__(self):
        self.list_one = [1, 2, 3]


class Bar(object):
    def init(self):
        self.list_two = Foo.list_one + [4, 5, 6] # I know this won't work unless I 
                                                 # make list_one a class level variable

So, the question is – is there any way to merge those lists without instantiation of Foo or making list_one a class level variable?

Maybe there is a way of doing so using inheritance and super() but I can’t understand how do I access variables inside methods with it. I also would be very grateful for an answer how to do it without using inheritance.

Answer 1

It’s impossible to do the exact thing you describe. self is a reference to an instance of Foo, and doesn’t exist until the class is instantiated; and list_one is an attribute of self, assigned to self after it has already been created. So list_one doesn’t exist at all until after the class has been instantiated.

To do what you’re asking using inheritance is fairly simple, however:

class Foo(object):
    def __init__(self):
        self.list_one = [1, 2, 3]

class Bar(Foo):
    def init(self):
        super(Bar, self).__init__()
        self.list_two = self.list_one + [4, 5, 6]
        # if you want to discard list_one, you can, but that seems wrong somehow
        # del self.list_one

More generally, it’s not really meaningful to refer to “variables inside methods” except during the execution of the method or function, or when referring to what’s called the closure created by the function. Every time a function is called, a new namespace is created, and local variable names are created anew inside that namespace.

It is possible to refer to them once the function has been called, though. For example:

>>> def foo(x):
...     a = x
...     def bar():
...         print a
...     return bar
... 
>>> bar1 = foo(5)
>>> bar2 = foo(6)
>>> bar1()
5
>>> bar2()
6

Observe that the two versions of bar refer to different values of a, each of which were created when foo(5) and foo(6) were called.