what is happening at __init__

[Eric Brunel]

Hi,

Your questions regarding the so-called "constructor" may simply show that 
the term "constructor" is not so good after all. In fact, the constructor 
does not "build" anything: the instance is already created (i.e. allocated 
in the memory) when the constructor is called. So your Q2 starts on the 
wrong basis: self is *not* assigned to a new value within the function; the 
new value is created *before* the call to the constructor, which simply 
references it as its parameter named "self", just like any other 
function/method. So if you do something like:

class C:
  def __init__(self):
    self = None
o = C()

o will *not* be set to None, but to a new, uninitialized instance of C: as 
always in Python, modifications to a method parameter are lost when the 
method ends, and the "constructor" is no exception.

In fact, Python is a tad more explicit than other languages when it calls 
the "constructor" __init__, because it would be far more accurate to call 
it an "initializer" rather than a "constructor": it doesn't create the 
instance, but simply initialize its attributes. And note it is not specific 
to Python: in C++ or in Java, the "constructor" is also called when the 
instance has already been allocated, so it doesn't build it.

Maybe this enlightens you a little bit more. The answers for your other 
questions should now be clear:
- Q1 is a "yes but": instance is created, then passed to the constructor 
where it is assigned to local variable "self"
- Q2 is pointless: self is *not* assigned in the constructor
- Q3 is no: the return value for the constructor is completely ignored, so 
you can return whatever you want (it's just useless...)
- Q4 is yes: you work on the actual instance that was created before the 
call to the constructor
- Q5 is yes: it is not the return value of the constructor that matters, 
but what is done to self's attributes within the method

If there are still unclear points in all that, you should carefully read 
the section 3.1 of the Python language reference manual ("Objects, values 
and types") where the objects and references concepts are described (IMHO 
not so well, but it's a start...).

HTH
 - eric -