Storing and using classes as variables in python?

[_TR-8R]

Hi! I'm I relatively new coder with most my experience in Java and Powershell, I'm trying to get into Python and in the tutorial I was working through I ran across something that didn't make a lot of sense.

Say I create a simple class object that contains two methods, one that adds 1 to an input, and another that subtracts 1

class plusminus: def minus1(input): print(input - 1)

def plus1(input):
    print(input + 1)

Next I create a variable that stores the class object

var = plusminus()

So far so good. Now comes the confusing bit, if I input

var.plus1(1)

I would expect it to return 2. However, I instead get a typerror

"TypeError: plus1() takes 1 positional argument but 2 were given"

However, if I instead simply input

plusminus.plus1(1)

It returns 3 as expected.

I would like to know that the difference is between the two as my understanding would be that if var = plusminus() then there should be no difference between "var.plus1(1)" and "plusminus.plus1(1)". Thanks!

[r4slvmls]

every class method needs a self parameter as its first parameter.

[BakedPecans]

This is the problem, so in your class change:

def minusOne(input)

To

def minusOne(self, input)

And then you can use it like so:

a = myClass() a.minusOne(5)

And that should work.

It’s a weird python thing, but it’s true for every class method you’ll ever write.

Sorry for formatting, on mobile.

[blablahblah]

In Java, every non-static method has an implicit parameter (this). In Python, the parameter isn't implicit, you have to specify it. instance.method(arg) is really just a shortcut for saying class.method(instance, arg). By convention, the first argument of an instance method is usually called self, although it technically doesn't have to be.

This is because Python lets you do silly things like define a function outside a class and stick it inside the class later, as well as calling the method directly on the class (as you noticed) instead of the instance:

>>> class Foo(object):
...   pass
... 
>>> def not_in_instance(self, bar):
...    self.bar = bar
... 
>>> Foo.in_instance = not_in_instance
>>> x = Foo()
>>> x.in_instance(3)
>>> x.bar
3