Why updating “shallow” copy dictionary doesn’t update “original” dictionary? [duplicate]

By “shallow copying” it means the content of the dictionary is not copied by value, but just creating a new reference.

>>> a = {1: [1,2,3]}
>>> b = a.copy()
>>> a, b
({1: [1, 2, 3]}, {1: [1, 2, 3]})
>>> a[1].append(4)
>>> a, b
({1: [1, 2, 3, 4]}, {1: [1, 2, 3, 4]})

In contrast, a deep copy will copy all contents by value.

>>> import copy
>>> c = copy.deepcopy(a)
>>> a, c
({1: [1, 2, 3, 4]}, {1: [1, 2, 3, 4]})
>>> a[1].append(5)
>>> a, c
({1: [1, 2, 3, 4, 5]}, {1: [1, 2, 3, 4]})

So:

1. b = a: Reference assignment, Make a and b points to the same object.

   

2. b = a.copy(): Shallow copying, a and b will become two isolated objects, but their contents still share the same reference

   

3. b = copy.deepcopy(a): Deep copying, a and b‘s structure and content become completely isolated.