Python Basics: Understanding Assignment, Shallow Copy, and Deep Copy for Strings and Lists

Assignment in Python is just creating a new reference (alias) to the same object.

Example:

list1 = [12,31,4,5565,34]
list2 = list1
print(list1)
print(list2)
list1[2] = "哈哈"
print(list1)
print(list2)

The output shows both list1 and list2 reflect the change because they refer to the same list.

To avoid this, a shallow copy can be used for one‑dimensional lists.

import copy
list3 = [23,45,6,78,93,42]
list4 = list3.copy()
print(list3, list4)
list3[3] = "呵呵"
print(list3)
print(list4)

The output shows list4 remains unchanged, confirming that shallow copy creates a new list object with the same elements.

For multi‑dimensional lists, shallow copy is insufficient; a deep copy is needed.

list5 = [23,43,[45,52,47],98,94]
import copy
list6 = copy.deepcopy(list5)
print(list5, list6)
list5[2][1] = "张三"
print(list5)
print(list6)

The output demonstrates that modifications to nested elements affect only the original list, while the deep‑copied list stays unchanged.