python - best practise for using variables in class and functions

Question

I am writing a large amount of code and I haven't got enough experience on the best way of using variables in a python class and its functions so I thought I would ask for some opinions. Here is an example of how I could do it

class MyClass():
    vara = 'Class Variable A'

    def __init__(self):
        self.varb = 'Class instantiation Variable B'

    def a(self):
        varc = 2
        vard = 3

        print 'def a function variables unchanged', varc, vard

        MyClass.vara = 'Class Variable A now changed by def a'

    def b(self):
        print self.varb
        self.varb = 'Class instantiation Variable B now changed by def b'
        print self.varb

if __name__ == "__main__":
    print MyClass.vara

    a = MyClass()
    a.a()
    a.b()

    print MyClass.vara

    a.a()

This gives this result

Class Variable A
def a function variables unchanged 2 3
Class instantiation Variable B
Class instantiation Variable B now changed by def b
Class Variable A now changed by def a
def a function variables unchanged 2 3

My app in django is using a class with has around 25 functions. Its a test application and each function represents an API call. Each API call will give out a few variables based on the result and I need these variables set by the functions to be used by other functions - so in other words I need to call these local function variables by other functions. I want to know the best way to do this.

In the example above, I can set the class variable vara using a function within the class, by doing this MyClass.vara = 'something' or I can set the variable which is in the instantiation constructor, __init__. Then I can call these variables using any function.

What's the better way, or is there an even better way to do this?

BrenBarn · Accepted Answer · 2012-08-31 14:31:12Z

4

Not sure if I fully get what you're asking, but if I understand you correctly, the answer is that it depends what you want to do. You should set the attribute on the class (as in your vara) if you want it to be stored once for the entire class. You should set it on the instance in __init__(as in your varb) if you want a separate attribute for each instance.

The difference becomes clearer if you have a mutable object (like a list) as the value of the attribute. See below:

>>> class Foo(object):
...     classVar = []
...     def __init__(self):
...         self.instVar = []
>>> f = Foo()
>>> g = Foo()
>>> f.classVar
[]
>>> f.instVar
[]
>>> g.classVar
[]
>>> g.instVar
[]
>>> f.instVar.append(1)
>>> f.instVar
[1]
>>> g.instVar
[]
>>> f.classVar.append(1)
>>> f.classVar
[1]
>>> g.classVar
[1]

Notice that in the last bit, mutating f.classVar also affected g.classVar. Because this is a class attribute, it is shared among all instances of the class. The instance attribute (instVar) is independent for each instance.

answered Aug 31, 2012 at 14:31

BrenBarn

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1 Comment

Oli Over a year ago

ah ok.. I think I get you. So i can create many instances using an object. The object.classvariable will be the same regardless if either object changes the class variable, but the instant variable for the class will be independent to each object?

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