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?
This question is related to
python
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.
Source: Stackoverflow.com