Variables inside and outside of a class __init__() function
I noticed that in Python, people initialize their class attributes in two different ways.
The first way is like this:
class MyClass: __element1 = 123 __element2 = "this is Africa" def __init__(self): #pass or something else
The other style looks like:
class MyClass: def __init__(self): self.__element1 = 123 self.__element2 = "this is Africa"
Which is the correct way to initialize class attributes?
Neither way is necessarily correct or incorrect, they are just two different kinds of class elements:
__init__method are static elements; they belong to the class.
__init__method are elements of the object (
self); they don't belong to the class.
You'll see it more clearly with some code:
class MyClass: static_elem = 123 def __init__(self): self.object_elem = 456 c1 = MyClass() c2 = MyClass() # Initial values of both elements >>> print c1.static_elem, c1.object_elem 123 456 >>> print c2.static_elem, c2.object_elem 123 456 # Nothing new so far ... # Let's try changing the static element MyClass.static_elem = 999 >>> print c1.static_elem, c1.object_elem 999 456 >>> print c2.static_elem, c2.object_elem 999 456 # Now, let's try changing the object element c1.object_elem = 888 >>> print c1.static_elem, c1.object_elem 999 888 >>> print c2.static_elem, c2.object_elem 999 456
As you can see, when we changed the class element, it changed for both objects. But, when we changed the object element, the other object remained unchanged.
I think this sample explains the difference between the styles:
james@bodacious-wired:~$cat test.py #!/usr/bin/env python class MyClass: element1 = "Hello" def __init__(self): self.element2 = "World" obj = MyClass() print dir(MyClass) print "--" print dir(obj) print "--" print obj.element1 print obj.element2 print MyClass.element1 + " " + MyClass.element2 james@bodacious-wired:~$./test.py ['__doc__', '__init__', '__module__', 'element1'] -- ['__doc__', '__init__', '__module__', 'element1', 'element2'] -- Hello World Hello Traceback (most recent call last): File "./test.py", line 17, in <module> print MyClass.element2 AttributeError: class MyClass has no attribute 'element2'
element1 is bound to the class, element2 is bound to an instance of the class.