Collections

Copying Collections

Copies can duplicate the outer container while nested objects may still be shared.

Assignment does not copy a collection. It gives the same list another name.

Source

rows = [["Ada"], ["Grace"]]
alias = rows

print(alias is rows)

Output

True

A shallow copy creates a new outer list, but nested lists are still shared.

Source

shallow = rows.copy()
rows[0].append("Lovelace")

print(shallow is rows)
print(rows[0] is shallow[0])
print(shallow)

Output

False
True
[['Ada', 'Lovelace'], ['Grace']]

A deep copy is independent at the nested level, so later mutation of rows[0] does not appear in deep.

Source

import copy

rows = [["Ada"], ["Grace"]]
deep = copy.deepcopy(rows)
rows[0].append("Lovelace")

print(rows[0] is deep[0])
print(deep)

Output

False
[['Ada'], ['Grace']]

Notes

Assignment aliases; it does not copy.
Shallow copies duplicate the outer container only.
Deep copies are useful for nested independence, but they can be expensive and surprising for objects with external resources.

Run the complete example

True
False
True
False
[['Ada', 'Lovelace'], ['Grace']]
[['Ada'], ['Grace']]

Execution time appears here after you run the example.