copy.copy() produces a shallow copy of an object. When the object's class defines copy, copy.copy calls that method and uses its return value as the copy. If copy is not defined, copy.copy falls back to the default behavior: for most objects, it creates a new instance and copies the dict (shallowly). For objects with slots or custom reduce_ex, the default may differ. Defining copy lets you control exactly what is copied, which is useful when the object holds resources, caches, or references that should not be duplicated. The risk is that a poorly implemented copy can break expectations, for example by returning self or by not copying mutable state. Always ensure copy returns a new object of the same type with the desired shallow-copy semantics.

1
With copy: copy.copy calls obj.copy() and returns its result.
2
Without copy: copy.copy uses the default shallow copy, typically copying dict.
3
Shallow copy means nested objects are shared, not duplicated.
4
Define copy when you need to control which attributes are copied or to reset caches.
5
Define deepcopy for deep copies, which otherwise uses pickle-like recursion.
6
Common mistake: returning self from copy, which does not create a copy.
7
Common mistake: forgetting to copy mutable attributes in copy, leading to shared state.
8
Version note: the copy module and copy protocol have been stable since Python 2.

Scenario Questions

0-2 years experience

1What does copy.copy do if a class does not define __copy__?
2Does a shallow copy duplicate nested lists?

2-5 years experience

1You define __copy__ and return self. What is the problem?
2You need a copy of an object but want to reset its cache. How do you use __copy__?

5-8 years experience

1You have a class with a database connection and you want copy.copy to create a new object without the connection. How do you implement __copy__?
2A custom __copy__ breaks deepcopy. How do you implement both correctly?

8+ years experience

1Design a copy protocol for a complex object graph that supports both shallow and deep copies with resource management.
2Explain how copy.copy, __reduce_ex__, and pickling interact for objects with custom state.

Follow-up Questions

What is the difference between __copy__ and __deepcopy__?
How does copy.copy behave with __slots__?

Sharethis question

Share via WhatsApp, X, Facebook, LinkedIn or copy link. Open Graph preview enabled.

What is the difference between copy.copy() behavior on a custom object with copy defined versus one without it?

Difficulty: 9/10

Copy, Shallow Copy, Dunder Methods, Object Protocol

copy.copy uses copy if defined; otherwise falls back to default shallow copy

1
With copy: copy.copy calls obj.copy() and returns its result.
2
Without copy: copy.copy uses the default shallow copy, typically copying dict.
3
Shallow copy means nested objects are shared, not duplicated.
4
Define copy when you need to control which attributes are copied or to reset caches.
5
Define deepcopy for deep copies, which otherwise uses pickle-like recursion.
6
Common mistake: returning self from copy, which does not create a copy.
7
Common mistake: forgetting to copy mutable attributes in copy, leading to shared state.
8
Version note: the copy module and copy protocol have been stable since Python 2.