calling a function from class in python - differen

Question

EDIT2: Thank you all for your help! EDIT: on adding @staticmethod, it works. However I am still wondering why i am getting a type error here.

I have just started OOPS and am completely new to it. I have a very basic question regarding the different ways I can call a function from a class. I have a testClass.py file with the code:

class MathsOperations:
    def __init__ (self, x, y):
        self.a = x
        self.b = y
    def testAddition (self):
        return (self.a + self.b)

    def testMultiplication (self):
        return (self.a * self.b)

I am calling this class from another file called main.py with the following code:

from testClass import MathsOperations

xyz = MathsOperations(2, 3)
print xyz.testAddition()

This works without any issues. However, I wanted to use the class in a much simpler way.

I have now put the following code in the testClass.py file. I have dropped the init function this time.

class MathsOperations:
    def testAddition (x, y):
        return x + y

    def testMultiplication (a, b):
        return a * b

calling this using;

from testClass import MathsOperations
xyz = MathsOperations()
print xyz.testAddition(2, 3)

this doesn't works. Can someone explain what is happening wrongly in case 2? How do I use this class?

The error i get is "TypeError: testAddition() takes exactly 2 arguments (3 given)"

Answer 1

Your methods don't refer to an object (that is, self), so you should use the @staticmethod decorator:

class MathsOperations:
    @staticmethod
    def testAddition (x, y):
        return x + y

    @staticmethod
    def testMultiplication (a, b):
        return a * b

Answer 2

you have to use self as the first parameters of a method

in the second case you should use

class MathOperations:
    def testAddition (self,x, y):
        return x + y

    def testMultiplication (self,a, b):
        return a * b

and in your code you could do the following

tmp = MathOperations
print tmp.testAddition(2,3)

if you use the class without instantiating a variable first

print MathOperation.testAddtion(2,3)

it gives you an error "TypeError: unbound method"

if you want to do that you will need the @staticmethod decorator

For example:

class MathsOperations:
    @staticmethod
    def testAddition (x, y):
        return x + y

    @staticmethod
    def testMultiplication (a, b):
        return a * b

then in your code you could use

print MathsOperations.testAddition(2,3)

Answer 3

You need to have an instance of a class to use its methods. Or if you don't need to access any of classes' variables (not static parameters) then you can define the method as static and it can be used even if the class isn't instantiated. Just add @staticmethod decorator to your methods.

class MathsOperations:
    @staticmethod
    def testAddition (x, y):
        return x + y
    @staticmethod
    def testMultiplication (a, b):
        return a * b

docs: http://docs.python.org/library/functions.html#staticmethod

Answer 4

class MathsOperations:
    def __init__ (self, x, y):
        self.a = x
        self.b = y
    def testAddition (self):
        return (self.a + self.b)

    def testMultiplication (self):
        return (self.a * self.b)

then

temp = MathsOperations()
print(temp.testAddition())

Answer 5

disclaimer: this is not a just to the point answer, it's more like a piece of advice, even if the answer can be found on the references

IMHO: object oriented programming in Python sucks quite a lot.

The method dispatching is not very straightforward, you need to know about bound/unbound instance/class (and static!) methods; you can have multiple inheritance and need to deal with legacy and new style classes (yours was old style) and know how the MRO works, properties...

In brief: too complex, with lots of things happening under the hood. Let me even say, it is unpythonic, as there are many different ways to achieve the same things.

My advice: use OOP only when it's really useful. Usually this means writing classes that implement well known protocols and integrate seamlessly with the rest of the system. Do not create lots of classes just for the sake of writing object oriented code.

Take a good read to this pages:

• http://docs.python.org/reference/datamodel.html
• http://docs.python.org/tutorial/classes.html

you'll find them quite useful.

If you really want to learn OOP, I'd suggest starting with a more conventional language, like Java. It's not half as fun as Python, but it's more predictable.