Python Copying Lists: Assignment vs Slicing vs copy()

A task list is copied for a second employee.

What You Need to Understand

Assignment copies a reference, not a list. Slicing with [:] and list.copy() create a new outer list but share nested objects. Understanding the difference prevents changes from appearing in unexpected places.

When to Use It

A task list is copied for a second employee.

Step-by-Step Python Example

tasks = ["read", "write"]
alias = tasks
copy_of_tasks = tasks.copy()
alias.append("test")
print(tasks)
print(copy_of_tasks)
print(tasks is alias, tasks is copy_of_tasks)

How the Example Works

The alias points at the same list and sees the appended value; the separate copied list does not.

Example Output

['read', 'write', 'test']
['read', 'write']
True False
True False

A Second Example

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

source = [1, 2]
copy_by_slice = source[:]
print(source == copy_by_slice, source is copy_by_slice)

Important Behavior and Edge Cases

This is only a shallow copy. Copying a list of lists preserves references to its inner lists.

Try It Yourself

Try slice copying and compare object identity with is.

Common Beginner Mistakes

This is only a shallow copy. Copying a list of lists preserves references to its inner lists.
Overlooking how an iterable is consumed, how element positions are chosen, or whether a function creates a new list.

Key Takeaways

Assignment copies a reference, not a list.
The alias points at the same list and sees the appended value; the separate copied list does not.
This is only a shallow copy.