What is the difference between a shallow copy and a deep copy in Python?
When duplicating complex objects, like lists or dictionaries that contain other objects, understanding the difference between a shallow copy and a deep copy is crucial. These terms describe how the inner objects are copied, and their distinction can have significant implications for how your program operates.
- Shallow Copy:
A shallow copy of an object is a new object that is a duplicate of the original object along with references to the objects found in the original. In essence, it copies the outer container, but the contents (or inner objects) are still references to the same objects as in the original. Therefore, if the copied container (like a list) is modified, the original remains unaffected. However, if the contents of the container (like individual elements of a list) are altered, those changes will reflect in both the original and the copied objects because they both refer to the same inner objects.
- Deep Copy:
A deep copy creates a new object that’s a recursive duplicate of the original along with all the objects inside it. This means both the outer container and all the inner contents are fully duplicated. Changes to the copied object, be it to the container itself or its contents, won’t affect the original object and vice versa. They are, in every practical sense, two entirely separate objects.
The distinction between shallow and deep copies becomes essential when dealing with mutable objects like lists or dictionaries nested inside another object. Using a shallow copy when a deep copy is needed, or vice versa, can lead to unintended side effects, including shared state or excessive memory usage.
- How to Make Copies in Python:
Python’s standard library provides the `copy` module to facilitate both types of copies. The `copy()` function produces a shallow copy, while the `deepcopy()` function, as the name suggests, creates a deep copy.
Understanding the difference between shallow and deep copying is foundational when managing complex objects in Python. While a shallow copy duplicates the outer container with references to the same inner objects, a deep copy ensures a full, recursive duplication, creating two entirely independent objects. Choosing the right copying method ensures predictable behavior and robustness in your applications.