Must all Python instance variables be declared in def __init__?

Question

Or can they be declared otherwise?

The code below does not work:

class BinaryNode():
    self.parent = None
    self.left_child = None

Answer 1

Nope. I love the @property variable for just this thing:

class Data(object):
    """give me some data, and I'll give you more"""
    def __init__(self, some, others):
        self.some   = some
        self.others = others

    @property
    def more(self):
        """you don't instantiate this with __init__, per say..."""
        return zip(self.some, self.others)


>>> mydata = Data([1, 2, 3], ['a', 'b', 'c'])
>>> mydata.more
[(1, 'a'), (2, 'b'), (3, 'c')]