Question

I’m still getting my head around classes in Python,

class Triangle(object):
    def __init__(self, angle1, angle2, angle3):
        angle1 = self.angle1
        angle2 = self.angle2
        angle3 = self.angle3
    number_of_sides = 3
    def check_angles(angle1, angle2, angle3):
        if angle1+angle2+angle3 == 180:
            return True
        else:
            return False

my_triangle = Triangle(30, 60, 90)
print(Triangle.number_of_sides)
print(Triangle.check_angles)

So what’s wrong?

Stackoverflow has some really mean users, I come from MathStackExchange and people didn’t downvote novice/noob questions as much as they do here.

Answer 1

You have your assignments backward:

self.angle1 = angle1

etc.

Thinking about what is going on a little bit might help here. self is an instance of the class Triangle that is passed in. So,

T = Triangle()
T.method_call()  #basically the same as `Triangle.method_call(T)`

Within method_call, self is a reference to T. (__init__ is kind of magic — It gets called automatically on the first line, but the principle is still the same). Once you know that, it’s easy to see why you need self.whatever = whatever — You’re putting a new attribute on the object T!
Finally, this explains how you should write check_angles:

def check_angles(self):
    #remember, `self` is a reference to a triangle instance -- T in our hypothetical code
    #so we can use `self` to gain access to all the attributes we set in `__init__`.
    if self.angle1 + self.angle2 + self.angle3 == 180:
        return True
    else:
        return False

---

Now a quick diversion into class attributes. You can put attributes onto a class as well:

class Triangle:
    number_of_sides = 3

I think it is customary to define class attributes before you define your methods (functions), but you don’t actually have to. (It will help the readers of your code understand it though — I misunderstood your original code because of the order you did things).

You can dynamically add attributes to a class after the class has been created as well:

class Triangle:
    ...

Triangle.number_of_angles = Rhombus.number_of_angles - 1

You can get access to Triangle.some_attribute a few ways. The first way is directly through the class (Triangle.number_of_sides). The second way is via an instance:

T = Triangle()
T.number_of_sides

This seems a little funny at first. After all, the instance doesn’t have a number_of_sides attribute. It turns out that python is designed to look at the instance first to see if it has it’s own number_of_sides attribute. If it does, great, that’s what you get. If it doesn’t however, python will then look at the class for the attribute. This turns into a handy way to share data between instances of a class.