Python List: Create, Change and Copy a Packing List

Start with a small, explicit task

Build a packing list for two people. First let both names refer to the same flat list, then give one person a copy. Finally use a nested packing list to see whether copying the outer container also copies the inner one. Predict which original entries will change before running the checks.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_list.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

shared = ["map", "bottle"]
partner = shared
partner[0] = "compass"
independent = shared.copy()
independent[1] = "flask"
nested = [["snack"], ["jacket"]]
shallow = nested.copy()
shallow[0][0] = "fruit"
assert partner is shared
assert independent is not shared
assert shared == ["compass", "bottle"]
assert independent == ["compass", "flask"]
assert nested == [["fruit"], ["jacket"]]
print(shared)
print(independent)
print(nested)

Explain the result

The first replacement is visible through shared because partner is another name for that same object. The independent list has its own outer slots, so changing its second slot does not replace the corresponding slot in shared. Equality compares contents; is compares identity, which is why the example checks both concepts.

The nested result is different: shallow and nested contain references to the same inner lists. Replacing an item inside the first inner list therefore affects both views. A shallow copy is useful when that sharing is intended. If your model requires independent nested objects, define and check that deeper copying requirement explicitly.

Avoid these mistakes

Do not treat a copy as a universal fix for every shared-state design. Decide which objects should remain shared. Keep list-method questions separate too: adding an item, removing a value and removing an index are different tasks, covered in the related tutorials.