Question

Should I write

class MyClass:
     def __init__(self):
         self.field = 0

or

class MyClass:
     def __init__(self):
         field = 0

Is there any difference between these statements?

Answer 1

Yes, you must use the self variable to set the properties of Python objects/instances.

In the second case, you’re just creating a local variable in __init__, not defining a property.

class MyClass:
    def __init__(self):
        field = 0

print MyClass().field # an error! it's not defined!

You may be misled by the way that you can just use raw assignments in class blocks to set class properties:

class MyClass:
    a = 2

print MyClass.a # 2

The behaviour in class blocks is unusual, and you don’t get similar behaviour inside methods.