Understanding dict.copy() - shallow or deep?

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.

   

Take this example:

original = dict(a=1, b=2, c=dict(d=4, e=5))
new = original.copy()

Now let's change a value in the 'shallow' (first) level:

new['a'] = 10
# new = {'a': 10, 'b': 2, 'c': {'d': 4, 'e': 5}}
# original = {'a': 1, 'b': 2, 'c': {'d': 4, 'e': 5}}
# no change in original, since ['a'] is an immutable integer

Now let's change a value one level deeper:

new['c']['d'] = 40
# new = {'a': 10, 'b': 2, 'c': {'d': 40, 'e': 5}}
# original = {'a': 1, 'b': 2, 'c': {'d': 40, 'e': 5}}
# new['c'] points to the same original['d'] mutable dictionary, so it will be changed

It's not a matter of deep copy or shallow copy, none of what you're doing is deep copy.

Here:

>>> new = original 

you're creating a new reference to the the list/dict referenced by original.

while here:

>>> new = original.copy()
>>> # or
>>> new = list(original) # dict(original)

you're creating a new list/dict which is filled with a copy of the references of objects contained in the original container.

"new" and "original" are different dicts, that's why you can update just one of them.. The items are shallow-copied, not the dict itself.