Why am I getting a NameError when I try to access an attribute in my class?

Question

I have this code with a class:

class Triangle(object):
    def __init__(self, side1, side2, side3):
        self.side1 = side1
        self.side2 = side2
        self.side3 = side3

    def perimeter(self):
        return "Perimeter = %s" % (side1 + side2 + side3)

a = Triangle(3, 4, 5)
print(a.perimeter())

Running this code throws an exception:

Traceback (most recent call last):
  File "untitled.py", line 12, in <module>
    print(a.perimeter())
  File "untitled.py", line 9, in perimeter
    return "Perimeter = %s" % (side1 + side2 + side3)
NameError: name 'side1' is not defined

How come I can't access side1 in the perimeter method?

Answer 1

Replace

return "Perimeter = %s" %(side1 + side2 + side3)

To

return "Perimeter = %s" %(self.side1 + self.side2 + self.side3)

You missed to add self

Answer 2

This line:

return "Perimeter = %s" %(side1 + side2 + side3)

should be:

return "Perimeter = %s" %(self.side1 + self.side2 + self.side3)

To return the value of member variables in python, self. must be before the member. This is why self is one of the required parameters for member methods. In many other languages such as C#, the passing of self is implied, so you don't have to manually write it into the code.

Answer 3

Unlike many other languages, python does not allow implicit access to instance attributes. To access an attribute in python, you must explicitly prefix it with self. like this:

return "Perimeter = %s" % (self.side1 + self.side2 + self.side3)

Without the self. prefix, python considers side1 to be a local or global variable.

---

The same thing applies to methods. For example, if you wanted to call perimeter from __init__, this would not work:

def __init__(self, side1, side2, side3):
    self.side1 = side1
    self.side2 = side2
    self.side3 = side3

    perimeter()

Instead, you would have to write

self.perimeter()

See also this related question.