problem with "self" operator when inheriting from a class i wrote

Question

I defined a two classes within one file. I was able to have it run after separating the classes into two files, but I wanted to know why the first attempt does not work.

The traceback read:

Traceback (most recent call last):
  File "Polygon.py", line 15, in <module>
    class Triangle(Polygon):
  File "Polygon.py", line 21, in Triangle
    print(self.sides)
NameError: name 'self' is not defined

and the bare bones of my code was as follows

class Polygon:
    def __init__(self, no_of_sides):
        self.n = no_of_sides
        self.sides = [0 for i in range(no_of_sides)]
class Triangle(Polygon):
    def __init__(self):
        Polygon.__init__(self,3)

It threw the same NameError when I ran this as the body of class Triangle

class Triangle(Polygon):
    self.a = 1

Your traceback does not fit your code. The offending line does not exist. — Klaus D.
– Klaus D., Commented Oct 5, 2018 at 4:18

ksbg · Accepted Answer · 2018-10-05 04:19:14Z

1

It looks like what you are trying to do, is define a class attribute a in class Triangle. You don't need self to do that. self is used to access the object instance of a class.

If you would want to define a as an object attribute rather than a class attribute, you should do something like:

class Triangle(Polygon):
    def __init__(self):
        Polygon.__init__(self, 3)
        self.a = 1

In this case, a is only defined upon class instantiation and accessible as Triangle().a, or as self.a inside other object-scope methods.

And if you want to define a as a class attribute, simply do:

class Triangle(Polygon):
    a = 1

    def __init__(self):
        ...

In this case, a is defined upon class definition and accessible even before instantiation, as Triangle.a

edited Oct 5, 2018 at 4:19

answered Oct 5, 2018 at 4:12

ksbg

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