[Tutor] Properties of an object

[Vicent]

This is an easy question, I guess, but I am not able to find out the answer.

In fact, it is both a Python question and a general programming "style"
question.

I want to define a class that contains a list (or a NumPy array) of elements
of a certain type, and an integer:

class ExampleList :
    def __init__(self, list, position) :
        self.list = list
        self.position = position


This is clear to me.

My first question, which is Python related:

(1) Where are the right places to define PROPERTIES for my class, and how (I
mean, which is the right syntax) must I define them?

As far as I know, the usual place where properties are "declared" and
assigned is in the   __init__   function.

Can I define properties in other parts of the "Class" code? I am not able to
deduce it from the things I've read.

I've also seen sometimes something like this:


class ExampleFoo :
    _property1 = 0

    [...]

And I think that later I found "self._property1" or "self.property1", but I
am not sure of that. Anyway, why the prefix "_", and what does that
definition "_property = 0" mean and imply?


Coming back to my ExampleList, I have another "philosophic" question.

Suppose I'm interested (many times within my "main" program) to recover a
part of the list contained by an ExampleList object. To be more clear:

>>> a = ExampleList([1,2,3,4], 2)
>>> a.list
[1, 2, 3, 4]
>>> a.position
2

I want to get the second part of the list, I mean, the part of list
a.list     that goes from position   a.position    until the last element in
the list.

I know I can do it like this:

>>> a.list[a.position:]
[3, 4]

Or, if I want the first part of the list:

>>> a.list[:a.position]
[1, 2]


My problem/question/dilemma:  I want to define it as a property or as a
method in the class definition, because I am going to need many times "the
first part of te list" or "the second part of the list".

For me, I see it as a property of the object, I mean: "tell me which is your
first part". It is not like an action over the object or to/from/related to
the object (that would be a method, then).

So, this kind of "derived" properties (because they are based on two other
previously defined "primary" properties) are better to be defined as
methods, or it is the same?

I mean, what I would like to do is this:

class ExampleList :
    def __init__(self, list, position) :
        self.list = list
        self.position = position
        self.list_1stpart = self.list[:self.position]
        self.list_2ndpart = self.list[self.position:]


Is that right? Is that good practice? If I want to define
self.list_1stpart   as a property, is the  __init__  function the only and
the best place to do it?

If not, I suppose I'll have to define it as a method, and then it will look
like something like this:

    [... within the class definition ...]

    def list_1stpart(self) :
        return self.list[:self.position]


Then I'll call the method like this:

>>> a.list_1stpart()
[1, 2]

For me, the "()" look like artificial, not necessary. I would prefer just to
type    "a.list_1stpart"   , a property.

The general case is (the second question):

(2) properties which can be derived from other "primary" properties, are
they just methods??

I don't know which approach is more correct, from any point of view... By
the way, does the "property" approach consume much memory or space than the
"method" approach??

Thank you very much for your patience...


-- 
Vicent
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