How is Python – .format(self=self) used?

I see a piece of code that looks like this:

class Car:
    def __init__(self,color,mileage):
        self.color = color
        self.mileage = mileage

def __str__(self):
        return 'a {self.color} car'.format(self=self)

my_car = Car('blue', 13585)
print(my_car)

How does self=self used in the str method work?

def __str__(self):
        return 'a {self.color} car'.format(self=self)

The first self in self=self refers to what the format function will change in your string. For example, it can also be

'a {obj.color} car'.format(obj=self)

Self on the left in self=self refers to the actual value that will be entered. In this case, the object is passed as a parameter. In other words, it can also be

def __str__(obj):
        return 'a {self.color} car'.format(self=obj)

So, for the overall View, you have

def __str__(value_passed):
        return 'a {value_to_change.color} car'.format(value_to_change=value_passed)

Why use self now?

This is just a convention used in Python programming. Python automatically passes an object to its instance method, which is a pointer to itself. See also this question for more information