Class Attributes/Variables in Python

Question

Two examples;

class Foo1:
    something = 0
    def __init__(self, n):
        self.something = n

and

class Foo2:
    def __init__(self, n):
        self.something = n

Both classes seem to have the same behaviour:

x = Foo1(42)
y = Foo2(36)
print x.something
# will print 42
print y.something
# will print 36

But in the class Foo1 is the variable self.something (in the constructor) actually the variable something as defined at the beginning of the class? What is the difference here? Which way is preferred to use?

possible duplicate of Python: Difference between class and instance attributes — sebastian
– sebastian, Commented Oct 30, 2014 at 13:34
Also, check the answers at stackoverflow.com/questions/2923579/python-class-attribute and stackoverflow.com/questions/68645/…. Each instances of Foo1 can access (and share) something with x.__class__.something — fredtantini
– fredtantini, Commented Oct 30, 2014 at 13:34
Thank you. Sorry if this is a duplicate. I was missing the right terminology to look for an answer correctly. — ap0
– ap0, Commented Oct 30, 2014 at 13:36

Tamim Shahriar · Accepted Answer · 2014-10-30 13:32:11Z

3

The difference is, in the first case, you can access the data attribute without using an object (or instance). In the second case, you can't. Please check the following code snippet :

>>> class Foo1:
...     something = 0
...     def __init__(self, n):
...         self.something = n
... 
>>> Foo1.something
0
>>> class Foo2:
...     def __init__(self, n):
...         self.something = n
... 
>>> Foo2.something
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class Foo2 has no attribute 'something'
>>>

Hope I could clarify. For more, please read this : https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables

answered Oct 30, 2014 at 13:32

Tamim Shahriar

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2 Comments

ap0 Over a year ago

Would this be pythons equivalent to static variables in C?

Tamim Shahriar Over a year ago

I don't think C static variable concept is required for Python.

anon582847382 · Accepted Answer · 2014-10-30 13:33:29Z

2

something is not the same as self.something. When you access x.something or y.something, you are accessing the version that is bound to that instance. The original something is local to that type rather than an object from that type:

>>> Foo1.something
0
>>> Foo1(42).something
42

answered Oct 30, 2014 at 13:33

anon582847382

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