python – How to copy a dictionary and only edit the copy

Assignment statements in Python do not copy objects, they create bindings between a target and an object.

so, dict2 = dict1, it results another binding between dict2and the object that dict1 refer to.

if you want to copy a dict, you can use the copy module.
The copy module has two interface:

copy.copy(x)
Return a shallow copy of x.

copy.deepcopy(x)
Return a deep copy of x.

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

For example, in python 2.7.9:

>>> import copy
>>> a = [1,2,3,4,['a', 'b']]
>>> b = a
>>> c = copy.copy(a)
>>> d = copy.deepcopy(a)
>>> a.append(5)
>>> a[4].append('c')

and the result is:

>>> a
[1, 2, 3, 4, ['a', 'b', 'c'], 5]
>>> b
[1, 2, 3, 4, ['a', 'b', 'c'], 5]
>>> c
[1, 2, 3, 4, ['a', 'b', 'c']]
>>> d
[1, 2, 3, 4, ['a', 'b']]

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