Copying Dictionaries0%
Dictionary Methods

Copying Dictionaries

Beginner8 min read•Updated: 2026-09-03
Study Materials

Copying Dictionaries

Avoid mutation bugs by understanding how Python duplicates dictionaries in memory.

Key Concepts & Detailed Explanation

Copying mechanisms:

  1. 1
    Reference Assignment (d2 = d1): Does NOT copy! Both point to the exact same dictionary in memory. Modifying d2 changes d1.
  2. 2
    Shallow Copy (`d2 = d1.copy()"): Copies top-level keys. If a value is a nested list or dict, it is still shared!
  3. 3
    Deep Copy (`copy.deepcopy(d1)"): Recursively duplicates every nested structure. Completely independent.

Code Examples & Output

Python
import copy
 
original = {
"id": 1,
"scores": [80, 90]
}
 
# Shallow copy vs Deep copy
shallow = original.copy()
deep = copy.deepcopy(original)
 
# Modify nested list
original["scores"].append(100)
 
print("Original scores:", original["scores"]) # [80, 90, 100]
print("Shallow scores (affected):", shallow["scores"]) # [80, 90, 100]
print("Deep scores (protected):", deep["scores"]) # [80, 90]

Expected Output:

Output
Original scores: [80, 90, 100]
Shallow scores (affected): [80, 90, 100]
Deep scores (protected): [80, 90]

Best Practices & Common Pitfalls

Always use copy.deepcopy() when cloning configurations or nested data models.

Practice Quiz

1. Does 'dict2 = dict1' create a new independent dictionary?

  • A) Yes
  • B) No, it merely copies the memory reference
  • C) Only in functions
  • D) Only if small
Answer: B Explanation: Assignment copies references, not the underlying object.

2. Which module provides the deepcopy() function?

  • A) sys
  • B) copy
  • C) math
  • D) os
Answer: B Explanation: The standard 'copy' module provides deepcopy().

Practice Challenge

Demonstrate that modifying a shallow-copied nested dictionary alters the original, but deepcopy protects it.

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