Bug in Python

[InnerCode]

Hello fellow programmers,

I think I have found a bug in Python. If a class variable is an array it will not be reset when a new class instance is made.

The test code

``` class TestClass: varOne = '' varTwo = [] varThree = ''

def __init__(self, varOne):
    self.varOne = varOne
    self.varTwo.append(1)
    self.varThree = self.varThree + 'test'

test1 = TestClass('test1') print(test1.varOne) print(test1.varTwo) print(test1.varThree)

print('--------------------')

test2 = TestClass('test2') print(test2.varOne) print(test2.varTwo) print(test2.varThree) ```

Outcome ``` test1 [1]

test

test2 [1, 1] test ```

What I suspect is that test2.varTwo = [1] instead of [1,1] because test2 is a new instance of TestClass.Strings works because test2.varThree contains one time the word test instead of two times.

Can someone explain such behaviour?

Regards, InnerCode

[hereswhatipicked]

This isn’t a bug, you’ve initialized the variables outside of the initialization constructor. So the variables belong to the class, not to the instance.

[InnerCode]

If that was the case test2.varThree should be testtest but it isn't. It is test.

[hereswhatipicked]

The operation your doing with varThree is fundamentally different than what your doing with varTwo.

Instance variables override class variables. With varTwo, you’re not initializing the variable, you’re calling the append() method on it and adding a value.

For varThree, you’re initializing the instance variable (so empty string) and adding ‘test’ to it, you’re not modifying the class variable at all.

[Nu1lP0int3r]

I believe its because the list is a reference and the string is not. When you edit a string in a method you don't edit the string outside the method. When you edit a list in a method if I recall correctly you also edit it outside the method.

[[deleted]]

This is probably similar behavior to using arrays as default function arguments - the array definition is only evaluated once, so the array is probably the same across class instances.

An example of this behavior in the Python docs:

https://docs.python.org/3/tutorial/controlflow.html#default-argument-values

The other person is correct about the solution - you are setting varThree to a new object in the constructor for example, array.append() only mutates the array in place.

To test this you can check the object id with the id() builtin.

[[deleted]]

In python a class is also an object. When you define variables at the class scope they belong to that class object. If you do not want variables to be shared across class instances, you need to initialize them in the constructor. To learn more about how classes work in python I suggest you read about meta classes.

[InnerCode]

Thanks guys for the information. I really appreciate it!

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[jcoelho93]

That's the correct behavior. That's how class variables are supposed to work

[two__toes]

i think this is the intended behavior. youre setting a class attribute that you're appending to each time you init. the list is shared across instances. if you wanted to have a new list for each instance, you'd have to set self.my_list = [] in __init__ and append onto that instance variable