Access parent class instance attribute from child class instance?

Question

How to access "myvar" from "child" in this code example:

class Parent():
    def __init__(self):
        self.myvar = 1

class Child(Parent):
    def __init__(self):
        Parent.__init__(self)

        # this won't work
        Parent.myvar

child = Child()

Solution 1

Parent is a class - blue print not an instance of it, in OOPS to access attributes of an object it requires instance of the same, Here self/child is instance while Parent/Child are classes...

see the answer below, may clarify your doubts.

class Parent():
    def __init__(self):
        self.myvar = 1

class Child(Parent):
    def __init__(self):
        Parent.__init__(self)

        # here you can access myvar like below.
        print self.myvar

child = Child()
print child.myvar

Solution 2

Parent does not have an attribute called myvar. Only instances of parent have that attribute. From within a method of Child, you can access that attribute with self.myvar.

Solution 3

Alternative to using inheritance

The current answers are coming from an inheritance perspective, but this isn't always what you want -- sometimes you might want the child to be an entirely different type of object to the parent, but that still has access to the parent attributes.

For a business analogue, think of Excel Workbooks which have Worksheet children, which themselves have Range children, and so on.

Only Child

An alternative approach (and not the only one) is to pass the parent as an argument to the child to create a property that corresponds to the parent:

class Parent(object):
    def __init__(self, parent_value):
        self.parent_value = parent_value
        self.child = Child(self)


class Child(object):
    def __init__(self, _parent):
        self.parent = _parent
        self.child_value = 0


new_parent = Parent(1)
print(new_parent.parent_value)        # prints 1

new_child = new_parent.child
print(new_child.child_value)          # prints 0
print(new_child.parent.parent_value)  # prints 1

new_parent.parent_value = 100
print(new_child.parent.parent_value)  # prints 100

Note that this instantiates the child at the same that that new_parent is instantiated. To access the parent's attributes, just go through the parent property.

Multiple Children

You could extend this so that you can create multiple instances of the Child class through the new_parent object. The code below is one simple way of doing this which replaces the child property with a children property and an add_child method.

class Parent(object):
    def __init__(self, parent_value):
        self.parent_value = parent_value
        self.children = []

    def add_child(self, child_value):
        new_child = Child(child_value, _parent=self)
        self.children.append(new_child)
        return new_child  # allows add_child to assign a variable


class Child(object):
    def __init__(self, child_value, _parent):
        self.parent = _parent
        self.child_value = child_value


new_parent = Parent(1)

# add 3 Child instances with child_values 2, 4, 8
[new_parent.add_child(v) for v in [2, 4, 8]]

# add another child that utilises the return statement
extra_child = new_parent.add_child(16)

for child in new_parent.children:
    print(child.child_value)          # prints 2, 4, 8, 16
    print(child.parent.parent_value)  # prints 1

new_parent.parent_value = 32
for child in new_parent.children:
    print(child.parent.parent_value)  # prints 32

# prove that extra_child is new_parent.children[3]
extra_child.child_value = 64
print(new_parent.children[3].child_value)  # prints 64

Comments

[ihadanny]

if your attribute starts with a 'dunder' (e.g. self.__myvar) it won't work as python considers it private

[Admin]

I could have sworn I tried the normal "self" instance reference. Oh well. Waiting to accept answer...

[Ahmed Alhallag]

Note: in this case, you don't need to have an init method in the subclass for this solution to work.

[Bill Wallis]

For future readers, the super() method is preferred in the child class __init__ instead of the parent class name. See this question

[Peilonrayz]

Whilst your answer is good quality, I don't think your answer answers the question as is. The question asks about inheritance not about composition or container classes. And I honestly don't think the OP needs to employ composition here.