Python Shallow vs Deep Copies of Nested Lists

Two teams need independent copies of a checklist with sublists.

What You Need to Understand

A shallow copy duplicates only the outer collection, while a deep copy recursively copies nested objects where possible. This distinction matters when a data structure includes lists within lists or dictionaries.

When to Use It

Two teams need independent copies of a checklist with sublists.

Step-by-Step Python Example

import copy
original = [["draft"], ["review"]]
shallow = original.copy()
deep = copy.deepcopy(original)
original[0].append("approved")
print(shallow[0])
print(deep[0])

How the Example Works

The shallow copy shares the inner list and sees approved. The deep copy keeps an independent inner list.

Example Output

['draft', 'approved']
['draft']
False

A Second Example

Compare this smaller variation with the main example, then experiment with your own values.

import copy
records = [{"done": False}]
clone = copy.deepcopy(records)
records[0]["done"] = True
print(clone[0]["done"])

Important Behavior and Edge Cases

Deep copying can be expensive and may not be meaningful for open files, connections or other resource objects. Prefer simpler data design when possible.

Try It Yourself

Create two lists of dictionaries and compare changes after shallow and deep copying.

Common Beginner Mistakes

Deep copying can be expensive and may not be meaningful for open files, connections or other resource objects. Prefer simpler data design when possible.
Overlooking how an iterable is consumed, how element positions are chosen, or whether a function creates a new list.

Key Takeaways

A shallow copy duplicates only the outer collection, while a deep copy recursively copies nested objects where possible.
The shallow copy shares the inner list and sees approved.
Deep copying can be expensive and may not be meaningful for open files, connections or other resource objects.