📝 How to Copy a Dictionary in Python – The Right Way | TechTown.in

In Python, dictionaries are mutable, and copying them correctly is crucial to avoid unexpected bugs in your code. If you simply assign one dictionary to another, both variables will point to the same memory reference — which means changes in one affect the other!

In this post, we’ll explore the correct ways to copy dictionaries in Python, including .copy() method and the dict() constructor — plus a bonus tip on copying nested dictionaries.

⚠️ The Wrong Way: Using `=`

original = {"name": "Tanmay", "age": 22}
copy_dict = original

copy_dict["age"] = 25
print(original)  # {'name': 'Tanmay', 'age': 25}

🎯 Both original and copy_dict refer to the same object in memory.

✅ Method 1: Using `.copy()`

original = {"name": "Tanmay", "age": 22}
copy_dict = original.copy()

copy_dict["age"] = 25
print(original)   # {'name': 'Tanmay', 'age': 22}
print(copy_dict)  # {'name': 'Tanmay', 'age': 25}

✅ Safe and reliable — this creates a shallow copy.

✅ Method 2: Using `dict()` Constructor

original = {"brand": "Apple", "product": "iPhone"}
copy_dict = dict(original)

Just like .copy(), this also creates a shallow copy.

🤔 What’s a Shallow Copy?

A shallow copy copies only the outer dictionary — not nested ones.

student = {
    "name": "Aditi",
    "marks": {"math": 90, "science": 88}
}

copy_student = student.copy()
copy_student["marks"]["math"] = 100

print(student["marks"]["math"])  # ❗Output: 100

🧠 Changes inside nested structures reflect in both.

🛡 Bonus: Deep Copy (for Nested Dictionaries)

For a true, independent copy of nested structures, use Python’s copy module:

import copy

deep_copy_student = copy.deepcopy(student)
deep_copy_student["marks"]["math"] = 70
print(student["marks"]["math"])  # ✅ Still 100

📝 Summary – Dictionary Copying Methods

Method	Type of Copy	Nested Objects Affected?	Use When…
`=` assignment	Reference only	✅ Yes	You want both variables to sync
`.copy()`	Shallow copy	❌ No (except nested items)	Copy top-level only
`dict()` constructor	Shallow copy	❌ No	Clean alternative to `.copy()`
`copy.deepcopy()`	Deep copy	✅ No	You need full isolation

🏁 Final Thoughts

Copying a dictionary isn’t just about duplication — it’s about control and safety. Whether you’re handling user data, configurations, or backups, using the right copy method ensures your changes don’t cause side effects elsewhere.

🧪 Tip: Always prefer .copy() or dict() for shallow copies, and go for deepcopy() when dealing with nested data.

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Copy Dictionaries