问题
Maybe I don't understand the definition of shallow copy... but i'm very confused:
from the docs:
Where "s" is a list (but same question applies to dictionaries respectively).
"s.copy() | creates a shallow copy of s (same as s[:])"
Except I thought s[:] was a deep copy. For example, see this stack overflow answer on how to copy a list (without it just pointing to the original version). And using list1.copy() seems to do a deep copy as well aka same behaviour as [:]
l1 = [1,2,3,4]
l2 = l1[:]
l3 = l1.copy()
l2.append(5)
l3[0] = 99
print(l1)
print(l2)
print(l3)
>> [1,2,3,4]
>> [1,2,3,4,5]
>> [99,2,3,4]
It would appear that l1,l2, and l3 are all separate objects. What am I missing?
回答1:
You've simply misunderstood what "shallow" and "deep" mean in this context.
A shallow copy is a copy of the top level of elements only. If any of those elements are themselves lists, the copies will still refer to the original lists. This is what both l1[:] and l1.copy() do.
A deep copy is a copy at all levels. If any of the elements are lists, they will also be deep copied. No references will be shared. This is what copy.deepcopy() does.
回答2:
A shallow copy means that the new list holds references to the same object that the old list has.
For example:
foo = [1, 2, []]
bar = foo.copy()
bar[-1].append(3)
print(foo)
We'll see that mutations of objects in bar also "pollute" foo.
If we re-do this example using a deep copy, it's a different story:
import copy
foo = [1, 2, []]
bar = copy.deepcopy(foo)
bar[-1].append(3)
print(foo)
This is because the deep copy creates a new (deep copy) of the list instead of just copying over a reference to the old list.
来源：https://stackoverflow.com/questions/43983907/python-list-copy-shallow-vs-deep-copy