Direct assignment of the python, shallow vs. deep copy

• Direct assignment: in fact, object reference (alias).

• Shallow copy (copy): sub-objects inside a copy of the parent object, the object will not be copied.

• Deep copy (DeepCopy):  DeepCopy method copy module, complete copy of the parent object and child objects.

 

Direct assignment : a and b point to the same object

Shallow copy: b = a.copy ():  shallow copy, a and b are a separate object, sub-object, or they point to a unified object (reference).

Such as:

import copy
a = [1, {1,2,3}]
b = copy.copy(a)

a.append(2)

print(a, b)
#([1, set([1, 2, 3]), 2], [1, set([1, 2, 3])])
a[1].add(4)
print(a, b)
# ([1, set([1, 2, 3, 4]), 2], [1, set([1, 2, 3, 4])])

In the code shown above, after a.append (2), because a, b are independent objects, so a.append (2) does not affect the value of b. So the results print (a, b) are:

([1, set([1, 2, 3]), 2], [1, set([1, 2, 3])])。

However, a [1] and b [1] for the same object, a [1] .add (4) also affect b. So the results print (a, b) are:

([1, set([1, 2, 3, 4]), 2], [1, set([1, 2, 3, 4])])。

Deep copy: A complete copy of b and parent object and its children, the two are completely separate.

The sample above into shallow copy:

import copy
a = [1, {1,2,3}]
b = copy.deepcopy(a)

a.append(2)

print(a, b)

a[1].add(4)
print(a, b)

Results two print (a, b) are:

([1, set([1, 2, 3]), 2], [1, set([1, 2, 3])])
([1, set([1, 2, 3, 4]), 2], [1, set([1, 2, 3])])

可以发现对a,b的修改，完全不会影响b的值。