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I'm having a problem understanding how class / instance variables work in Python. I don't understand why when I try this code the list variable seems to be a class variable
class testClass(): list = [] def __init__(self): self.list.append('thing') p = testClass() print p.list f = testClass() print f.list Output:
['thing'] ['thing', 'thing'] and when I do this it seems to be an instance variable
class testClass(): def __init__(self): self.list = [] self.list.append('thing') p = testClass() print p.list f = testClass() print f.list Output:
['thing'] ['thing'] 
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A class variable is a variable that defines a particular property or attribute for a class. An instance variable is a variable whose value is specified to the Instance and shared among different instances. We can share these variables between class and its subclasses. We cannot share these variables between classes.
Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block. Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.
Class variable − A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class's methods. Class variables are not used as frequently as instance variables are.
What is an Instance Variable in Python? If the value of a variable varies from object to object, then such variables are called instance variables. For every object, a separate copy of the instance variable will be created. Instance variables are not shared by objects.
This is because of the way Python resolves names with the .. When you write self.list the Python runtime tries to resolve the list name first by looking for it in the instance object, and if it is not found there, then in the class instance.
Let's look into it step by step
self.list.append(1) list name into the object self? list name into the class instance of object self? But when you bind a name things are different:
self.list = [] list name into the object self? So, that is always an instance variable.
Your first example creates a list into the class instance, as this is the active scope at the time (no self anywhere). But your second example creates a list explicitly in the scope of self.
More interesting would be the example:
class testClass(): list = ['foo'] def __init__(self): self.list = [] self.list.append('thing') x = testClass() print x.list print testClass.list del x.list print x.list That will print:
['thing'] ['foo'] ['foo'] The moment you delete the instance name the class name is visible through the self reference.
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