Python 3.5 handling lists strangely [duplicate]

Question

I came across some rather strange behavior.

class Example:
    test = []

    def __init__(self):
        print(self.test)
        self.test.append(0)


ex1 = Example()
ex2 = Example()
ex3 = Example()

I'd expect this to output [] every time, however, I get:

[]
[0]
[0, 0]

What is this wizardry? Could you help me understand?
Thank, you!

Edit: Hey, thank you for the quick answers.

Just to clarify, if "test" is static then why do I not notice this behavior when I replace "self.test.append(0)" with "self.test = [0]"?

test is a class attribute here, not an attribute of each instance. Don't try to "declare" members like this; you must create them in __init__. — Jonathon Reinhart
– Jonathon Reinhart, Commented Aug 19, 2017 at 12:59
test is a class variable, not an instance variable. It is shared by all instances of your class. See, e.g., digitalocean.com/community/tutorials/…. — larsks
– larsks, Commented Aug 19, 2017 at 12:59
«why do I not notice this behavior when I replace "self.test.append(0)" with "self.test = [0]"?» Because that binds a new object to the instance attribute self.test, and that will shadow the Example.test class attribute. Whereas doing self.test.append(0) merely mutates the existing object. — PM 2Ring
– PM 2Ring, Commented Aug 19, 2017 at 13:14
BTW, please try to avoid modifying questions after they've already received valid answers. If you need additional clarification about an answer, do it in the comments of that answer, or ask a fresh question (possibly linking to the original question). Otherwise, you risk invalidating those answers & the answerers may not notice. — PM 2Ring
– PM 2Ring, Commented Aug 19, 2017 at 13:17

damd · Accepted Answer · 2017-08-19 13:09:06Z

2

test is a static class attribute, which you are continually updating with values. Python is different from some other languages in this way. To make it an object attribute, use self.test = [] in your constructor.

edited Aug 19, 2017 at 13:09

answered Aug 19, 2017 at 12:59

damd

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AKX · Accepted Answer · 2017-08-19 12:59:31Z

1

test there is a class-level static variable, which is shared between the instances of the class.

You'll want to initialize test within the __init__ method.

class Example:

    def __init__(self):
        self.test = []
        print(self.test)
        self.test.append(0)

answered Aug 19, 2017 at 12:59

AKX

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