CODEWITHSUNDEEP
python
matt2605
matt2605

Reputation: 216

How to use self in python

I'm new to programming and I want someone to explain using 'self' in python in the following context.

class Box:
    def __init__(self):
        self.length = 1.0
        self.width = 1.0
        self.height = 1.0    

    def set_dimensions(self, newL, newW, newH):
        self.length = newL
        self.width = newW
        self.height = newH          

    def volume(self):
        return (self.length * self.width * self.height)

box = Box:
box.set_dimensions(2.0,3.0,4.0)
print(box.volume())

This code causes an exception:

Error: box.set_dimensions(2.0,3.0,4.0) needs exactly 4 arguments, 3 given

Can someone explain how to use 'self' when calling methods please?

Upvotes: 1

Views: 532

Answers (3)

proski
proski

Reputation: 3949

If you write box = Box, you make box a variable referring to a class Box. It's very rare that you would need a variable to refer to a class. When calling method of a class, you need to supply an instance of that class as the first argument, but you haven't created any such instance.

Instead, write box = Box() - that would create and instance of the class Box. And then the remainder of the code would be valid. When calling a class method on a class instance, the instance is passed as an additional first argument, the one that is named self in the method definition.

Upvotes: 2

Endzior
Endzior

Reputation: 164

To add something to the answer, you can try to understand self in class function variables as something that when it is encoutnered python internally converts the objects method into calling it from the class, so when you are calling

SomeBoxObject.setDimensions(someLen, otherLen, evenOtherLen)

Python turns this into

Box.setDimensions(SomeBoxObject, someLen, otherLen, evenOtherLen)

Upvotes: 0

Bhargav Rao
Bhargav Rao

Reputation: 52181

Use parenthesis to create an instance of your class

box = Box()                       # Use parenthesis here not :
box.set_dimensions(2.0,3.0,4.0)   # Now no error
print(box.volume())               # Prints 24.0

Upvotes: 1

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