Copy and Deepcopy

Python · Reference cheat sheet

📋 Overview

Assignment binds names to objects; it does not copy. Use slicing/list()/copy.copy for shallow copies and copy.deepcopy for nested structures. Know when shared references are intentional.

🔧 Core concepts

Shallow: new container, same nested objects. Deep: nested containers cloned (with cycle handling).

💡 Examples

Alias vs shallow:

import copy

a = [[1], [2]]
alias = a
shallow = copy.copy(a)      # or a[:] / list(a)
alias[0].append(9)
print(a, shallow)           # both see [1, 9] in first row

Deepcopy:

import copy

a = [[1], [2]]
deep = copy.deepcopy(a)
a[0].append(9)
print(a)     # [[1, 9], [2]]
print(deep)  # [[1], [2]]

Dict / custom:

import copy

cfg = {"db": {"host": "localhost"}, "debug": True}
cfg2 = {**cfg}                 # shallow
cfg3 = copy.deepcopy(cfg)

class Node:
    def __init__(self, value: int) -> None:
        self.value = value
        self.child: Node | None = None

n = Node(1)
n.child = Node(2)
m = copy.deepcopy(n)

⚠️ Pitfalls

• list * n / [[0]*3]*3 creates shared inner rows — classic bug.
• Deepcopy can be slow and fail on locks, sockets, modules, or some C objects.
• Dataclass replace is shallow for nested mutables.
• Copying is not serialization — for disk/network use JSON/pickle carefully.
• __copy__ / __deepcopy__ can customize behavior on your classes.

🔗 Related

• Lists
• Dictionaries
• Dataclasses
• Slicing
• Types
• Scope / namespaces