Python OOP: how to use a flag parameter to access methods?

It's unclear to me how one accomplishes this in Python, though I am confused about the fundamentals of OOP:

Let's say I create a Shape class, and use this as an example.

class Shape:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def area(self):
        return self.x * self.y

    def perimeter(self):
        return 2 * self.x + 2 * self.y

    def scaleSize(self,scale):
        self.x = self.x * scale
        self.y = self.y * scale

In my case, I must create a Class which inputs a file whereby there are only two types, file_typeA and file_typeB. For the user, it would be exceptionally cumbersome to use a Class for Type A and a Class for Type B.

## use case for an a "read in file name" class `filename`
foo = filename(path = "path/to/file.txt", file_type = "TypeA")

My question is, how do I restrict the "Type A" methods for a file initialized with flag file_type="TypeA", and restrict the methods for a file initialized with flag file_type = "TypeB"?

Let's try the Shape class again, whereby the two types are Square and Triangle. The wrong way to code this is something like this:

class Shape:
    def __init__(self, x, y, type):
        self.x = x
        self.y = y
        self.type = type

    if type == "Square":

        def area(self):
            return self.x * self.y

        def perimeter(self):
            return 2 * self.x + 2 * self.y

        def scaleSize(self,scale):
            self.x = self.x * scale
            self.y = self.y * scale

    elif type == "Triangle":

        def hooray(self):
            print("HOORAY! It's hip to be a triangle!")

    else:
        print("'type' must be either Circle or Triangle")

 ## example Class instantiation
 bar = Shape(2, 3, type="Square")

For such a class, how do I create it such that the method area cannot be used for a Shape of type=="TypeB"?

Solution

I think you need to define a mother class:

class Shape:
   pass

then sub-classes

class Triangle(Shape):
   # whatever methods there are

What you're describing is a factory function which creates the proper object according to some parameter:

def create(type):
   if type=="Triangle":
       return Triangle()
   elif type=="Square":
       return Square()

but it's not very useful since the caller has to pass the name of the class instead:

c = create("Triangle")

instead of just doing:

c = Triangle()

Well, it may have its interest when saving/restoring a program state from a text file containing the type(s) of the objects in the program memory when saved (if you don't want to use pickle).