如何命名__init__用于确定新对象的值的方法？

Question

I'm new to Python, I started learning Python3.3 few days ago.

Let's have 2 classes, Source and Dest ; Dest is declared before Source .

I'd like to make a object belonging to class Dest with content according to on object belonging to class Source .

If it was C++ (which I know quite well), to do thas I could allow casting Source to Dest by writing Source::operator Dest() , operator Dest(Source) , Dest::Dest(Source) , operator Dest(Source&) or Dest::Dest(Source&) .

I think about decorating Dest 's __init__ to set Dest 's object's value on value returned by passed argument's special_method (if exist), which should return object with attributes similar to Dest 's object.

In code it could look like that:

def allow_conversion(prev_init, special_method_name):
    def wrap(self, source, *args, **dic):
        try:
            #call getattr(source, special_method_name)(*args, **dic)
            #copy attributes of returned object (don't know how to implement)
        except Exception: #if getattr(source, special_method_name)
            pass          #doesn't exist or raised Exception
        finally:
            prev_init(source, *args, **dic) #original __init__ is always called
    return wrap

class Dest:
    @allow_conversion('special_method')
    def __init__(self):
        pass #do some initializing

I wonder how to name the special_method . Is there any habit?

Answer 1

If I understand your question correctly, you simply want to create an instance of Dest using an instance of Source . For that, you should define a class method in Dest which takes an instance of Source as an argument, and returns a properly initialized instance of Dest .

class Source:
    def __init__(self, x, y):
        self.x = x
        self.y = y

class Dest:
    def __init__(self, a):
        self.a = a

    @classmethod
    def from_source(cls, src):
        return Dest(src.x + src.y)


s = Source(5, 3)
d = Dest.from_source(s)
assert d.a == 8

All the logic for "converting" an instance of Source to an instance of Dest will reside in Dest.from_source .