Python 3.5 handling lists strangely

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, Aug 19 '17 at 12:59
`test` is a class variable, not an instance variable. It is shared by all instances of your class. See, e.g., https://www.digitalocean.com/community/tutorials/understanding-class-and-instance-variables-in-python-3. — larsks, Aug 19 '17 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, Aug 19 '17 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, Aug 19 '17 at 13:17

damd · Accepted Answer · 2017-08-19T13:09:06.967

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 '17 at 13:09

answered Aug 19 '17 at 12:59

damd

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score 1 · Answer 2 · answered Aug 19 '17 at 12:59

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)

Python 3.5 handling lists strangely

2 Answers2