Attributes initialization/declaration in Python class: where to place them?

Question

I was wondering what was the best practice for initializing object attributes in Python, in the body of the class or inside the __init__ function?

i.e.

class A(object):
 foo = None

vs

class A(object):
 def __init__(self):
 self.foo = None

Answer 1

If you want the attribute to be shared by all instances of the class, use a class attribute:

class A(object):
 foo = None

This causes ('foo',None) to be a (key,value) pair in A.__dict__.

If you want the attribute to be customizable on a per-instance basis, use an instance attribute:

class A(object):
 def __init__(self):
 self.foo = None

This causes ('foo',None) to be a (key,value) pair in a.__dict__ where a=A() is an instance of A.

• fortran: makes sense, I thought they were bounded to the instance in both cases... thanks :)
• unutbu: Python has a complicated set of attribute lookup rules. Without getting into descriptors, for this case it suffices to know a.__dict__ is searched for the foo key first, and if it is not found there, then A.__dict__ is searched. This is why you can use a.foo even if foo is a class attribute.
• Kat: Also noteworthy is that for class attributes, if you were to do a.foo = whatever, you'd actually end up creating a new instance attribute (which hides the class attribute). A.foo would be unchanged. It's kinda a gotcha, since you can access the value of A.foo with a.foo, but assigning to that would create a new instance attribute.

Answer 2

Attributes defined in the class definition are considered class variables (like static variables in Java), while those set in the initializer are instance attributes (note the difference between self.something = 1 and something = 1). See this question for more details, and this one for even more. There is not a lot of practical difference between these two cases, as the class-level definition gives the attribute a default value, but if you want to use some kind of logic to set an attribute before using an object instance you should do it in the __init__() method.