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
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.
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.
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.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.
