Parent class has one backing instance

Question

Suppose I have the following code:

class ParentClass():
    myList = []

    def __init__(self):
        print("Parent class constructor")

class ChildA(ParentClass):
    def __init__(self):
        print("Child A constructor")

    def add_elem(self, item):
        self.myList.append(item)
        print(f"Added item by Child A, current length: {len(self.myList)}")

class ChildB(ParentClass):
    def __init__(self):
        print("Child B constructor")

    def add_elem(self, item):
        self.myList.append(item)
        print(f"Added item by Child B, current length: {len(self.myList)}")

a = ChildA()
a.add_elem(1)
a.add_elem(1)

b = ChildB()
b.add_elem(1)

When I run it (Python 3.9.3), I get this output:

Child A constructor
Added item by Child A, current length: 1
Added item by Child A, current length: 2
Child B constructor
Added item by Child B, current length: 3

This suggests that the parent class that the children classes inherit from has one backing instance, and because I didn't explicitly set myList to [] in the constructor, it uses the same backing memory, even for two different children classes.

Is this assumption correct?
What are valid uses for this design? When I first discovered this behavior I thought it might be a language bug, but I feel like that's quite unlikely.

I worked around this by just explicitly setting myList to [] in the parent constructor, but I am bewildered by this behavior, as it was not the behavior I was expecting (I would expect the length of myList after the first call to ChildB to be 1).

Yes, there's only one `myList` – Mad Physicist Apr 17 '21 at 01:13 — Mad Physicist, Apr 17 '21 at 01:13
You're forgetting to call the parent constructor – Mad Physicist Apr 17 '21 at 01:14 — Mad Physicist, Apr 17 '21 at 01:14

score 1 · Accepted Answer · answered Apr 17 '21 at 01:13

1

I worked around this by just explicitly setting myList to [] in the parent constructor, but I am bewildered by this behavior, as it was not the behavior I was expecting.

That's correct. What you've done is define a variable on the class itself, not on instances of the class. The correct way to define instance variables is in the constructor.

class ParentClass():

    def __init__(self):
        print("Parent class constructor")
        self.myList = []

answered Apr 17 '21 at 01:13

Silvio Mayolo

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That won't help: the parent constructor is never called – Mad Physicist Apr 17 '21 at 01:13
1

I would just do `super().__init__(self)` or something though to call it in the children. – captainGeech Apr 17 '21 at 01:14
@Silvio Mayolo that makes sense, thank you for clarifying that! I forgot to distinguish class vs instance variables like that. – captainGeech Apr 17 '21 at 01:15

Parent class has one backing instance

1 Answers1