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Copy Dictionaries

Copying dictionaries in Python is an important concept because dictionaries are mutable. If copied incorrectly, changes in one dictionary can affect another unintentionally.
This topic explains all correct ways to copy dictionaries, including shallow copy vs deep copy, with clear examples—similar to GeeksforGeeks-style explanations.

Why Copying Dictionaries Is Important

When you assign one dictionary to another variable, both variables refer to the same object.
python
a = {"x": 1, "y": 2}
b = a

b["x"] = 100
print(a)
print(b)
Both dictionaries change because they point to the same memory location.

Method 1: Copy Dictionary Using copy()

Creates a shallow copy of the dictionary.

Syntax

python
new_dict = old_dict.copy()

Example

python
a = {"x": 1, "y": 2}
b = a.copy()

b["x"] = 100
print(a)
print(b)

Method 2: Copy Dictionary Using dict() Constructor

Another way to create a shallow copy.

Syntax

python
new_dict = dict(old_dict)

Example

python
a = {"name": "Amit", "age": 30}
b = dict(a)

b["age"] = 35
print(a)
print(b)

Method 3: Copy Dictionary Using Dictionary Comprehension

Creates a new dictionary manually.

Syntax

python
new_dict = {k: v for k, v in old_dict.items()}

Example

python
a = {"a": 1, "b": 2}
b = {k: v for k, v in a.items()}

b["a"] = 10
print(a)
print(b)

Shallow Copy Explained

A shallow copy copies only the outer dictionary. Nested objects are still shared.
python
a = {
    "name": "Jayesh",
    "skills": ["Python", "AWS"]
}

b = a.copy()
b["skills"].append("Docker")

print(a)
print(b)
Both dictionaries change because the list inside is shared.

Deep Copy Using copy.deepcopy()

A deep copy copies all nested objects, creating a fully independent dictionary.

Syntax

python
import copy
new_dict = copy.deepcopy(old_dict)

Example

python
import copy

a = {
    "name": "Jayesh",
    "skills": ["Python", "AWS"]
}

b = copy.deepcopy(a)
b["skills"].append("Docker")

print(a)
print(b)

Difference Between Shallow Copy and Deep Copy

FeatureShallow CopyDeep Copy
Copies outer dictionaryYesYes
Copies nested objectsNoYes
Shared referencesYesNo
Memory usageLessMore

Copy Dictionary Using Loop

python
a = {"x": 1, "y": 2}
b = {}

for key, value in a.items():
    b[key] = value

print(b)

Copy Dictionary with Nested Dictionary (Tricky Case)

python
a = {
    "student": {
        "name": "Amit",
        "age": 20
    }
}

b = a.copy()
b["student"]["age"] = 25

print(a)
print(b)
This happens due to shallow copy behavior.
Use deep copy to avoid this.

Common Mistakes

Using Assignment Instead of Copy

python
a = {"x": 1}
b = a
This does not create a copy.

Forgetting Deep Copy for Nested Data

python
b = a.copy()
Nested structures are still shared.

Best Practices

  • Use copy() for simple dictionaries
  • Use deepcopy() for nested dictionaries
  • Avoid assignment when copying is required
  • Understand reference behavior clearly

Summary

  • Dictionaries are mutable
  • Assignment creates reference, not copy
  • copy() and dict() create shallow copies
  • Nested objects remain shared in shallow copy
  • deepcopy() creates fully independent copy
  • Choose copy method based on data complexity