How to use self in python

Question

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?

proski · Accepted Answer · 2015-06-08 21:51:14Z

2

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.

edited Jun 8, 2015 at 21:51

answered Jun 6, 2015 at 1:56

proski

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Bhargav Rao · Accepted Answer · 2015-06-06 01:42:47Z

1

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

answered Jun 6, 2015 at 1:42

Bhargav Rao

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Endzior · Accepted Answer · 2015-06-06 01:49:05Z

0

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)

answered Jun 6, 2015 at 1:49

Endzior

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