__init__ explanation please

[Lie]

I've been in this Python mailing list for a few days, and I've noticed
several things here: There are too many fundamentalist!

Don't play stupid and all, don't be a fundamentalist. It might be true
that __init__ isn't a constructor and __new__ might be the constructor
(some people even claimed __new__ is also not a constructor).

>From the base definition of a constructor: constructor is the creator
of an object. In this case, __new__ is technically the constructor
while __init__ is an initializer.

However, it is also to be noted that __init__ is what makes an object
meaningful, and that makes it a constructor in a sense (while still
technically a constructor). Without initialization, an object is
meaningless, even if the definition of the initializer is to leave it
as it is.

Python creates object by doing something like this:
a = anObject(arg1, arg2, arg3)

These arguments is then passed to __new__ and __init__ for their
arguments in its sake of creating and initializing the object. Then
anObject() returns an instance of anObject.

>From an outsider's point of view, there is no difference between
__new__ and __init__ since they're "implementation details" (in other
languages, these are private functions[1] that is invisible to
outsiders, Python doesn't like privacy but the semantic of being
implementation detail still exist). For an outsider, there is
absolutely no need to know that __new__ and __init__ exists, they just
need to know anObject()'s arguments, which is the public view of the
constructor and initializer[2].

[1] Well, for fundamentalists: constructors aren't usually private
though, usually they're Friend or Protected Friend which prohibits
outsiders from calling it but allow other classes inheriting from it
to call them.
[2] In this sense, from outsider's POV anObject() is the constructor.


If you can't be convinced with this argument, then I'd give you
another that's a bit more Pythonic:
DUCK TYPING: If it looks like a duck, walks like a duck, and quacks
like a duck, it is a duck!

>From the class programmer's point of view, __init__ acts like an
object constructor in other languages, there is no significant
difference between __init__ and constructor in other languages. The
fact that __init__ works with side-effect as opposed to returning the
object is not a significant point and can be considered as an
implementation difference (I'm not aware of any major programming
language that returns an instance of itself in its return value
[except for Python]).

For example, in VB.NET, there is no doubt that Sub New() is a
constructor, despite New() works only by side effect, and returning
anything results in an error (since it is a Sub or a method in
Python's dictionary). Not only VB, C++ and C# also use side effect in
its constructors and doesn't return a value. In this sense, VB's New, C
++ constructor, and C# constructor is equal to Python's __init__, thus
the Duck Typing spirit applies here.