[Tutor] Concept related to python classes

[Manprit Singh]

Dear Sir,

This is a continuation mail.
I have just tried to  rewrite the program of finding the area of a
triangle, as per the explanation given in the previous mail.


class Triangle:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

    def area(self):
        s = (self.a + self.b + self.c) / 2
        return (s * (s - self.a) * (s - self.b) * (s - self.c))**0.5

    def resize(self, a1, b1, c1):
        self.a = a1
        self.b = b1
        self.c = c1


tri = Triangle(4, 4, 3)
print(tri.area())            # Gives the answer =  5.562148865321747
tri.resize(3, 4, 5)         # This allows you to change all arms of
triangle in one line
print(tri.area())            # # Gives the answer =  6.0

Regards
Manprit Singh



On Mon, Sep 7, 2020 at 1:46 PM Alan Gauld via Tutor <tutor at python.org>
wrote:

> On 07/09/2020 06:17, Manprit Singh wrote:
> > Dear Sir ,
> >
> > Consider a problem to find the area of a triangle , using classes .
> > self.a,  self.b, self.c are instance variables representing 3 arms of
> > triangle.
> > I need an instance variable self.area that represents the area of the
> > triangle.
>
> Do you really? If you can calculate the area on demand then
> you don't need an instance variable you just need a method.
> The more instance variables you store the bulkier your objects
> become and the easier it is to get those variables in an
> inconsistent state. Keep the instance variables to the
> minimum that you need to support the methods. Let users
> of your classes access  them via messages/methods.
>
>
> > class Area:
>
> Why have you called it class area? Surely the object here
> is the triangle so this should e class Triangle?
>
> >     def __init__(self, a, b, c):
> >         self.a = a
> >         self.b = b
> >         self.c = c
> >         self.calculate()
> >
> >     def calculate(self):
> >         s = (self.a + self.b + self.c) / 2
> >         self.area =  (s * (s - self.a) * (s - self.b) * (s -
> self.c))**0.5
>
> calculate() is really calculating the area of the triangle
> so this should be your self.area() method.
>
> > tri = Area(4, 4, 3)     # Object initialization
> > print(tri.area)           #Will print 5.562148865321747 (area when arms
> are
>
> That then becomes:
>
> tri = Triangle(4,4,3)
> print (tri.area())
>
> > My first question is -
> >
> > The way I have used self.area as instance variable inside class
> definition
> > .
> > The point is i haven't written self.area inside the __init__( ) .
> > Is it not necessary to place an instance variable inside __init__( ) ?
>
> No, you can put instance variable creation anywhere inside the class
> definition. In fact in Python you can even add instance variables to
> objects outside of the class:
>
> class C:
>    def __init__(self,x): self.x = 42
>
> c = C(7)
> c.y = 66
>
> print(c.x,c.y)   # prints 42, 66,
>
>
> But this is usually a bad idea. The c.y only exists in the c instance
> not in any  others you create and the methods of the class know nothing
> about it so will not use it.
>
> And this is also the problem with defining variables in non init
> methods. That variable will only exist after the method is called
> for the first time. The other methods will not be able to use it
> safely because they font know whether it has been created yet.
> That means writing a lot of try/except code into the other
> methods or having a variable that only gets used by one method
> which is suspect. So it is usually better to put all instance
> variables into init even if you don't yet know what value to
> store in them.
>
> > My second question is :
> > The way i have called self.calculate() inside the __init__( ) is ok ?
> > My idea behind this is, upon calling this function inside __init__( ),
> the
> > area
> > triangle for initial values of the arms will be calculated and the
> > calculated
> > value of the area will be assigned to instance variable self.area.
> > Is this idea correct thinking ?
>
> Yes, and this is how to get round the problem of initialising the
> new variable outside init. In practice you are including it in
> init, albeit indirectly. however this brings us back to the
> initial point, if you are just storing the result of a calculation
> why not just use the calculation as needed? There is a tiny
> performance advantage but unless you know for sure you need
> that advantage its better to use the method on demand.
>
> > My third question is :
> > As you can can see i have reassigned the values of length of arms as :
> > tri.a = 3
> > tri.b = 4
> > tri.c = 5
> > and after it tri.calculate() is called , and then tri.area gives the
> > desired output.
>
> See me code above, you don't need to store the area, just call the
> method area() and use it on demand. But in principle what you have
> done is fine. You might consider whether you always want to change all
> three values in which case you might write a convenience method called
> something like resize() (or maybe reinit()) such that you can apply all
> three changes in one line:
>
> tri = Triangle(4,4,3)
> print(tri.area())
> tri.resize(3,4,5)
> print(tri.area())
>
> > I have not used getters & setters as python do not promote use of getters
> > and setters unless otherwise these are necessarily required.
>
> Good :-)
>
>
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
>
>
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