While reading up the documentation for dict.copy()
, it says that it makes a shallow copy of the dictionary. Same goes for the book I am following (Beazley’s Python Reference), which says:
The m.copy() method makes a shallow
copy of the items contained in a
mapping object and places them in a
new mapping object.
Consider this:
>>> original = dict(a=1, b=2)
>>> new = original.copy()
>>> new.update({'c': 3})
>>> original
{'a': 1, 'b': 2}
>>> new
{'a': 1, 'c': 3, 'b': 2}
So I assumed this would update the value of original
(and add ‘c’: 3) also since I was doing a shallow copy. Like if you do it for a list:
>>> original = [1, 2, 3]
>>> new = original
>>> new.append(4)
>>> new, original
([1, 2, 3, 4], [1, 2, 3, 4])
This works as expected.
Since both are shallow copies, why is that the dict.copy()
doesn’t work as I expect it to? Or my understanding of shallow vs deep copying is flawed?
7 s
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:
-
b = a
: Reference assignment, Makea
andb
points to the same object. -
b = a.copy()
: Shallow copying,a
andb
will become two isolated objects, but their contents still share the same reference -
b = copy.deepcopy(a)
: Deep copying,a
andb
‘s structure and content become completely isolated.