Create child class instances from parent class instance, and call parent methods from child class instance

Question

I wish I could do 2 things:

• 1. Create as many times of the Child class from an instance of the Parent class (What I can do)
• 2. Call a method of the Parent class from an instance of the Child class (What I can't do)

To illustrate my problem, I created 2 instances of the class Parent to which I add an instance of the class Child to each.

from datetime import datetime, timedelta


class Child:

    def __init__(self):
        pass

    def ask_time(self):                     # This function doesn't work,
        return self.read_time()             # But I would like to be able to call here the "read_time()" method of the class "Parent"


class Parent:

    def __init__(self, name, minutes_fast):
        self.name = name
        self.minutes_fast = minutes_fast
        self.children = {}

    def add_child(self, name):              # Construct "Child" class instance from class "Parent" class instance
        self.children[name] = Child()       # Because of this line, I cannot inherit "class Child (Parent):"

    def get_child(self, name):
        if name not in self.children:
            self.add_child(name)
            return self.children[name]

    def read_time(self):
        current_time = datetime.now()
        delta = timedelta(minutes=self.minutes_fast)
        return (current_time + delta).strftime("%H:%M:%S")


# Add the Parent "James" who is 3 minutes early to his watch, and add him the child "John"
parent1 = Parent("James", 3)
child1 = parent1.get_child("John")

# Add the Parent "Michael" who is 1 minutes early to his watch, and add him the child "Matthew"
parent2 = Parent("Michael", 1)
child2 = parent2.get_child("Matthew")


print(parent1.read_time())
print(parent2.read_time())

In my use case, reading the time is the responsibility of the class Parent. So I added the read_time() method to this one. But an instance of the class Child must be able to request the time from the instance of the class Parent that created it. So I add the ask_time() method to the class Child which calls the read_time() method of the class Parent... Which does not work without inheriting between my classes (from the following way class Child(Parent):).

Which would allow me to do this, and what I now need to do.

print(child1.ask_time())
print(child2.ask_time())

But I don't see how to inherit when class Parent itself depends on the class Child? Thanks for your help !

Answer 1

You have confused functional dependency with class inheritance. You do not have to inherit read_time from Parent. In fact, your implementation shows that this is not sufficient. As you correctly designed this, read_time is an instance attribute: it makes no sense to call read_time without specifying which Parent instance should respond.

You need to give each child a reference to its parent. Include this in add_child:

def add_child(self, name):
    baby = Child(self, name)
    self.children[name] = baby

Change the Child initialization to use the appropraite info:

def __init__(self, parent, name):
    self.parent = parent
    self.name = name

It seems silly to have the child's name be a property only of the parent's list.

Now, when the child needs to ask the time, we have:

def ask_time(self):
    return self.parent.read_time()