Difference between dictionary unpacking operator vs copy.deepcopy?

Question

Time to time I need to deepcopy an object and sometimes it's a dictionary object so I use ** operator to copy it easily but I always wondered if deepcopy is any different or efficient in some sense. Which one is more efficient in terms of memory footprint, cpu time and does deepcopy iterate over like ** operator does?

import copy
myObj = {"some": "data", "to": "copy"}

# With using unpacking operator.
newObj1 = {**myObj}

# With using `copy.deepcopy` function.
newObj2 = copy.deepcopy(myObj)

score 3 · Accepted Answer · answered Nov 17 '20 at 15:33

3

Be careful; there's a big difference when those dictionary values are mutable, and ** does not behave equivalently to copy.deepcopy():

>>> import copy
>>> myObj = {"some": "data", "to": ["copy", "this"]}
>>> newObj1 = {**myObj}
>>> newObj2 = copy.deepcopy(myObj)
>>> myObj["to"][1] = "that"
>>> newObj1
{'some': 'data', 'to': ['copy', 'that']}
>>> newObj2
{'some': 'data', 'to': ['copy', 'this']}

As shown in PEP 448, using ** iterable unpacking is more akin to dict.copy() than copy.deepcopy().

answered Nov 17 '20 at 15:33

Brad Solomon

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So the real object does not get mutated when you change `newObj1` but if you change the main object it mutates the inherited objects in `newObj1` too, did I get it right? – Guven Degirmenci Nov 17 '20 at 18:19
@GuvenDegirmenci in this case, changing elements of `newObj1['to']` will mutate `myObj`. They both contain references to the same mutable list. – Brad Solomon Nov 17 '20 at 19:27
Looks like there is some odd problem: https://repl.it/talk/share/Odd-Behaviour/80345 – Guven Degirmenci Nov 17 '20 at 20:02
I guess this answer [here](https://stackoverflow.com/a/184745/7493063) explains the problem. – Guven Degirmenci Nov 17 '20 at 20:09

Difference between dictionary unpacking operator vs copy.deepcopy?

1 Answers1