In Python, Class variables are declared when a class is being constructed. They are not defined inside any methods of a class because of this only one copy of the static variable will be created and shared between all objects of the class.
Use class_name dot variable_name to access a class variable from a class method in Python. Use instance to access variables outside the class.
The __init__ method is the Python equivalent of the C++ constructor in an object-oriented approach. The __init__ function is called every time an object is created from a class. The __init__ method lets the class initialize the object's attributes and serves no other purpose. It is only used within classes.
To print the type of variable in Python, use the type() function and to print the variable, use the print() function. The type() is a built-in Python function that returns the variable's data type.
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.
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