Does it make a difference using self for a temporary variable in a Python method?

Question

I sometimes need to use temporary variables in method definitions that aren\'t used outside the method. Is there any difference in behaviour between using self.MyVaria

Answer 1

The first creates a lasting reference on the class instance, and will be available on the object outside the scope of your method. The latter creates a purely local reference, which will not be available outside the method. Which is better depends on the situation, but if it's actually intended to only be a temporary variable use a local (non-self) variable.

Case 1:

>>> foo = MyClass()
>>> foo.MyVariable
...
AttributeError: 'MyClass' object has no attribute 'MyVariable'
>>> foo.Hello('bar')
'Hello bar'
>>> foo.MyVariable
'Hello bar'

Case 2 is as above, except that MyVariable is still not an attribute of the object after calling Hello.

Answer 2

Use example 2. Using self. is only necessary if you would like to use your variable in another method of this class or would like to access it later on the instance.

Answer 3

I would do neither, I would just return "Hello " + Name, you are not using MyVariable anywhere so there is no need to create it and definitely no need to make it an attribute:

class MyClass:
    def hello(self, name):
        return  "Hello " + name

Answer 4

The first usage would surprise me if I were looking at your code since it would make me think the variable was being used somewhere else in the class.

Since the first is surprising and makes your intent harder to discern, I'd say the latter is more pythonic, and better in any language that works more or less the same way.