Attributes initialization/declaration in Python class: where to place them?
PythonCoding StyleAttributesPython Problem Overview
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
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
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.