Question

A question about python deepcopy and shallow copy.

the post at
What is the difference between a deep copy and a shallow copy?

cannot help me.

why e.g. 1 ‘s sum is 6 not 10 ?

e.g.1 :

kvps = { '1' : 1, '2' : 2 }
theCopy = kvps.copy()  # both point to the same mem location ? 
kvps['1'] = 5
sum = kvps['1'] + theCopy['1']
print sum

output sum is 6

e.g.2 :

aList = [1,2]
bList = [3,4]
kvps = { '1' : aList, '2' : bList }

theCopy = kvps.copy()  # both point to the same mem location ? 
kvps['1'][0] = 5
sum = kvps['1'][0] + theCopy['1'][0]

print sum

output sum is 10

e.g.3 :

import copy

aList = [1,2]
bList = [3,4]
kvps = { '1' : aList, '2' : bList }

theCopy = copy.deepcopy(kvps)
kvps['1'][0] = 5
sum = kvps['1'][0] + theCopy['1'][0]

print sum

output sum is 6.

Also , e.g. 4

kvps = { '1' : 1, '2' : 2 }    
theCopy = dict(kvps)  #  theCopy hold a reference to kvps ?     
kvps['1'] = 5  # should also change theCopy , right ?    
sum = kvps['1'] + theCopy['1']    
print kvps    
print theCopy    
print sum

its sum is 6 , if theCopy is a reference to kvps , it should be 10.

Answer 1

Shallow copy makes a copy of mutable objects in the top-level container. A deep copy makes a new instance of all mutable containers in the data structure.

“e.g. 2” results in 10 because you copy the dict on the outside, but the two lists inside are still the old lists, and lists can be changed in-place (they’re mutable).

Deep copy makes runs aList.copy(), bList.copy() and replaces the values in your dict with their copies.

---

e.g. 1 explained:

kvps = {'1': 1, '2': 2}
theCopy = kvps.copy()

# the above is equivalent to:
kvps = {'1': 1, '2': 2}
theCopy = {'1': 1, '2': 2}

When you apply this to e.g. 2:

kvps = {'1': aList, '2': bList}
theCopy = {'1': aList, '2': bList}

The list objects in both dicts are the same objects, so modifying one of the lists will be reflected in both dicts.

---

Doing a deep copy (e.g. 3) results in this:

kvps = {'1': aList, '2': bList}
theCopy = {'1': [1, 2], '2': [3, 4]}

This means both dicts have entirely different contents, and modifying one won’t modify the other.

---

e.g. 4 via dict() is equivalent to a shallow copy.