6.2 copy and References

In Python, variables usually hold references to objects, not the objects themselves. Understanding references, shallow copy, and deep copy is essential when working with nested data.

References

If you write b = a, Python does not create a new copy. It makes b refer to the same object as a.

info = dict(name="Alice", skills=["Python", "C"])
info_ref = info

info_ref["skills"].append("Java")
print(info)

Output:

{'name': 'Alice', 'skills': ['Python', 'C', 'Java']}

Because info and info_ref point to the same dictionary, a change through one variable is visible through the other.

Shallow Copy

A shallow copy copies the outer object, but not the nested child objects.

import copy

info = dict(name="Alice", skills=["Python", "C"])
info_copy = copy.copy(info)

info.pop("name")
info_copy["skills"].append("Java")

print(info)
print(info_copy)

Output:

{'skills': ['Python', 'C', 'Java']}
{'name': 'Alice', 'skills': ['Python', 'C', 'Java']}

The outer dictionaries are different, but the inner skills list is still shared.

Deep Copy

A deep copy recursively copies the parent object and its child objects.

import copy

info = dict(name="Alice", skills=["Python", "C"])
info_copy = copy.deepcopy(info)

info.pop("name")
info_copy["skills"].append("Java")

print(info)
print(info_copy)

Output:

{'skills': ['Python', 'C']}
{'name': 'Alice', 'skills': ['Python', 'C', 'Java']}

After deep copy, the two skills lists are no longer the same object.