Question: As you can see in the following code

```
def __str__(self):
        return f"User: {self.observer.pk} => Habit: {self.habit.pk}"

```

There's `=>`. What's the meaning of that?

Answer: 
The `__str__` method of a python class defines how that class will be converted to a string. I wrote a little example using your provided code to show that:

```
class TestHelper:

    def __init__(self, pk):
        self.pk = pk

class Test:

    def __init__(self, observer_pk, habit_pk):
        self.observer = TestHelper(observer_pk)
        self.habit = TestHelper(habit_pk)

    def __str__(self):
        return f"User: {self.observer.pk} => Habit: {self.habit.pk}"

t = Test(5, 8)
print(t)

```

If you run this code, the result is `User: 5 => Habit: 8` gets printed out, as the `print` method automatically converts its arguments to strings.
You could replace the '=>' with whatever you want, and that will be printed instead.
If you comment out the `__str__` method `<__main__.Test object at some_hex_address>` will be printed - this is what the default `__str__` method inherited from the `object` class does. You can convert your class to a string however you want - it is completely up to you
Methods with double undersocore like **str**, are called magic/dunder methods.

The '**str**' job - You can define string representation for Object.

f'blabla {some\_variable}' - this is f-string. You can access to variable in string using {} brackets.

lets assume that self.observer.pk is 12, and self.habit.pk is 2 so:

This code will define for objects to have string representation like 'User: 12 => Habit: 2' Thats the string.

Cheers
Fenrir