Python- unexplained behaviour of datetime

Question

I'm trying to understand a weird behavior when using datetime inside python's dictionary. Code below is written to explain. I expected that the example of variable a and b will act the same as self.x and self.y.

To be more clear: at the beginning self.x=1,self.y=2, later on self.x=self.y, and inside a while loop - self.x is getting incremented by 1. As expected, only self.x is reaching the value of 5, while self.y remains at 2 as shown in run result below.

BUT I'm looking for an explanation why it is not happening with a, b which both increase by datetime.timedelta(days=3) while only b is was coded to be incremented.

Any explaintion why ?

class TBD:
    def __init__(self):
        self.x = 1
        self.y = 2
        print(self.x, self.y)
        self.run_it()

    def run_it(self):
        self.x = self.y
        z = 0
        a = {'start': datetime.datetime.now(), 'end': datetime.datetime.now() + datetime.timedelta(days=1)}
        b = a

        while z < 3:
            self.x += 1

            z += 1
            b['end'] = b['end'] + datetime.timedelta(days=3)
            print(b['end'], a['end'])
            print('x=', self.x, 'y=', self.y)

Run results:

1 2
2018-05-15 16:26:58.101727 2018-05-15 16:26:58.101727
x= 3 y= 2
2018-05-18 16:26:58.101727 2018-05-18 16:26:58.101727
x= 4 y= 2
2018-05-21 16:26:58.101727 2018-05-21 16:26:58.101727
x= 5 y= 2

Process finished with exit code 0

`b = a` **does not** create a copy, and there's a fundamental difference between mutable objects like dictionaries and immutable objects like integers. *(Note this is nothing to do with datetime.)* — jonrsharpe, May 11 '18 at 13:50
This isn't really specific to classes or datetimes; you could do `x = {0:1}; y = x; x[0] = 2; print(y)` and see that mutating x also mutates y. — Kevin, May 11 '18 at 13:51

Python- unexplained behaviour of datetime

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