Day 11:Using += thinking it creates a new object

 

🐍 Python Mistakes Everyone Makes ❌

Day 11: Using += Thinking It Creates a New Object

Many Python beginners assume += always creates a new object.
That’s not always true.

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❌ The Mistake

a = [1, 2, 3]
b = a

a += [4]

print(b)

Output:

[1, 2, 3, 4]

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❌ Why this surprises people?

Because += modifies the object in place for mutable types like lists.

• a and b both reference the same list

• Using += changes the original object

• Both variables see the change

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✅ The Correct Understanding

a = [1, 2, 3]
b = a.copy()

a += [4]

print(b)

Output:

[1, 2, 3]

Now a and b are different objects.

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🧠 Simple Rule to Remember

• += on mutable objects → modifies in place

• += on immutable objects → creates a new object

Example:

x = 5
x += 1 # new integer object created

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✅ Key Takeaway

+= does not always create a new object.
Understanding mutability helps avoid unexpected bugs.