Can a class variable be created inside one of the class' methods in Python?

Question

Touching on the following question posted on SO, Python - why use "self" in a class?, I was wondering:

If I created a variable inside a method of a given class without using the "self.", will it be a class variable? And if this is not the case, why not?

So for instance if I have a class like so:

class A:
    def somefunction(self):
        x=1

Will x be a class variable?

You ought to read the [Python Tutorial](https://docs.python.org/3/tutorial/). — Stefano Sanfilippo, Jul 18 '14 at 15:01

RomanHotsiy · Accepted Answer · 2014-07-18T15:14:37.690

3

Actually no. Because of x is not class variable - it is local variable of method somefunction.

Class variable should be defined exactly in the class body:

class A:
    x = 1
    def somefunction(self):
        A.x += 1
        print A.x

BTW: even if you had defined classvariable correct, you would anyway got output:

2
2

because of in somefunction you're assigning x=1, so each time this function is run value of x is reset to 1 :)

edited Jul 18 '14 at 15:14

answered Jul 18 '14 at 14:59

RomanHotsiy

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I should actually rephrase my question to "can class variables be created in methods" – user1491307 Jul 18 '14 at 15:51
Possibly.., most likely yes. You actually shouldn't ask questions before searching it in Google. – RomanHotsiy Jul 18 '14 at 15:54
No, of course I have searched it, but with the added information I got from your answer/comment, the question can be rephrased to be more succinct and meaningful. – user1491307 Jul 18 '14 at 16:02
You changes made my answer confusing :( – RomanHotsiy Jul 18 '14 at 16:11
Oh sorry.. But I think your answer makes perfect sense, apart from the last few lines explaining my example, which i have deleted. As you have pointed out, the function gets called multiple times, so the variable is reset/recreated and I think there is another reason it wouldn't work, namely because _x_ is immutable, so it seems to me that it would be just confusing to have it in this question. – user1491307 Jul 18 '14 at 16:19

score 1 · Answer 2 · answered Jul 18 '14 at 14:59

Whenever you assign a variable in a Python function, it's created in the innermost local namespace by default. You aren't referring to a variable in a different namespace (like the A class) unless you explicitly declare it. So in your code, you're creating x in a local namespace, which is specific to that function - not even specific to that instance of A, since you didn't declare it as self.x. I believe that is why you get 2,2 instead of 2,3.

score 1 · Answer 3 · answered Jul 18 '14 at 15:05

1

No, your assumption is wrong. It is a local variable.

answered Jul 18 '14 at 15:05

Daniel Roseman

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Can a class variable be created inside one of the class' methods in Python?

3 Answers3